How to prove the triangle is isosceles and determine its area












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Let $P(a,b)$ be a point on the curve $y = frac{1}{x}$ in first quadrant and let the tangent line at $P$ intersect the $x$-axis at a point $A$. Show that triangle $AOP$ is isosceles and determine its area, where $O$ is the origin.



How do I start to attempt this?










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$endgroup$








  • 1




    $begingroup$
    Have you tried to find the tangent line in terms of $P$ yet?
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 14:18










  • $begingroup$
    Very good. So the slope of $PA$ is $-1/a^2$. Now if you also calculate the slope of $OP$ you should notice something which makes the triangle very symmetric.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:04










  • $begingroup$
    So essentially, the slope of $OP$ is just the inverse of $PA$? @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:10










  • $begingroup$
    Right, the additive inverse that is. Because $O$ and $A$ lie on the $x$-axis, it follows that $|PA|$=$|OP|$.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:12










  • $begingroup$
    I see now. Thank you so much for your assistance! @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:13
















0












$begingroup$


Let $P(a,b)$ be a point on the curve $y = frac{1}{x}$ in first quadrant and let the tangent line at $P$ intersect the $x$-axis at a point $A$. Show that triangle $AOP$ is isosceles and determine its area, where $O$ is the origin.



How do I start to attempt this?










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    Have you tried to find the tangent line in terms of $P$ yet?
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 14:18










  • $begingroup$
    Very good. So the slope of $PA$ is $-1/a^2$. Now if you also calculate the slope of $OP$ you should notice something which makes the triangle very symmetric.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:04










  • $begingroup$
    So essentially, the slope of $OP$ is just the inverse of $PA$? @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:10










  • $begingroup$
    Right, the additive inverse that is. Because $O$ and $A$ lie on the $x$-axis, it follows that $|PA|$=$|OP|$.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:12










  • $begingroup$
    I see now. Thank you so much for your assistance! @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:13














0












0








0





$begingroup$


Let $P(a,b)$ be a point on the curve $y = frac{1}{x}$ in first quadrant and let the tangent line at $P$ intersect the $x$-axis at a point $A$. Show that triangle $AOP$ is isosceles and determine its area, where $O$ is the origin.



How do I start to attempt this?










share|cite|improve this question











$endgroup$




Let $P(a,b)$ be a point on the curve $y = frac{1}{x}$ in first quadrant and let the tangent line at $P$ intersect the $x$-axis at a point $A$. Show that triangle $AOP$ is isosceles and determine its area, where $O$ is the origin.



How do I start to attempt this?







analytic-geometry plane-geometry






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share|cite|improve this question













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share|cite|improve this question








edited Dec 14 '18 at 14:05









amWhy

1




1










asked Dec 13 '18 at 14:14









Valèt QuasarValèt Quasar

32




32








  • 1




    $begingroup$
    Have you tried to find the tangent line in terms of $P$ yet?
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 14:18










  • $begingroup$
    Very good. So the slope of $PA$ is $-1/a^2$. Now if you also calculate the slope of $OP$ you should notice something which makes the triangle very symmetric.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:04










  • $begingroup$
    So essentially, the slope of $OP$ is just the inverse of $PA$? @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:10










  • $begingroup$
    Right, the additive inverse that is. Because $O$ and $A$ lie on the $x$-axis, it follows that $|PA|$=$|OP|$.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:12










  • $begingroup$
    I see now. Thank you so much for your assistance! @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:13














  • 1




    $begingroup$
    Have you tried to find the tangent line in terms of $P$ yet?
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 14:18










  • $begingroup$
    Very good. So the slope of $PA$ is $-1/a^2$. Now if you also calculate the slope of $OP$ you should notice something which makes the triangle very symmetric.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:04










  • $begingroup$
    So essentially, the slope of $OP$ is just the inverse of $PA$? @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:10










  • $begingroup$
    Right, the additive inverse that is. Because $O$ and $A$ lie on the $x$-axis, it follows that $|PA|$=$|OP|$.
    $endgroup$
    – SmileyCraft
    Dec 13 '18 at 15:12










  • $begingroup$
    I see now. Thank you so much for your assistance! @SmileyCraft
    $endgroup$
    – Valèt Quasar
    Dec 13 '18 at 15:13








1




1




$begingroup$
Have you tried to find the tangent line in terms of $P$ yet?
$endgroup$
– SmileyCraft
Dec 13 '18 at 14:18




$begingroup$
Have you tried to find the tangent line in terms of $P$ yet?
$endgroup$
– SmileyCraft
Dec 13 '18 at 14:18












$begingroup$
Very good. So the slope of $PA$ is $-1/a^2$. Now if you also calculate the slope of $OP$ you should notice something which makes the triangle very symmetric.
$endgroup$
– SmileyCraft
Dec 13 '18 at 15:04




$begingroup$
Very good. So the slope of $PA$ is $-1/a^2$. Now if you also calculate the slope of $OP$ you should notice something which makes the triangle very symmetric.
$endgroup$
– SmileyCraft
Dec 13 '18 at 15:04












$begingroup$
So essentially, the slope of $OP$ is just the inverse of $PA$? @SmileyCraft
$endgroup$
– Valèt Quasar
Dec 13 '18 at 15:10




$begingroup$
So essentially, the slope of $OP$ is just the inverse of $PA$? @SmileyCraft
$endgroup$
– Valèt Quasar
Dec 13 '18 at 15:10












$begingroup$
Right, the additive inverse that is. Because $O$ and $A$ lie on the $x$-axis, it follows that $|PA|$=$|OP|$.
$endgroup$
– SmileyCraft
Dec 13 '18 at 15:12




$begingroup$
Right, the additive inverse that is. Because $O$ and $A$ lie on the $x$-axis, it follows that $|PA|$=$|OP|$.
$endgroup$
– SmileyCraft
Dec 13 '18 at 15:12












$begingroup$
I see now. Thank you so much for your assistance! @SmileyCraft
$endgroup$
– Valèt Quasar
Dec 13 '18 at 15:13




$begingroup$
I see now. Thank you so much for your assistance! @SmileyCraft
$endgroup$
– Valèt Quasar
Dec 13 '18 at 15:13










3 Answers
3






active

oldest

votes


















0












$begingroup$

To start approaching the problem, first notice that if P lies on the curve then $P (a, b) = P (a, frac{1}{a})$.



Secondly, find the slope of the tangent line at $P$, which is precisely $-frac{1}{a^2}$



Thus equation of the tangent line through $P$ is given by



$$(y - frac{1}{a}) = (-frac{1}{a^2})(x-a) implies a^2y = 2a - x$$



The above line intersects the $x$-axis only at $(2a, 0)$, thus $ A = (2a, 0)$.



Therefore the coordinates of $Delta AOP$ are precisely: $$A (2a, 0), O(0, 0), P (a, frac{1}{a})$$
making $AOP$ isosceles. Moreover, the area of $Delta AOP = frac{1}{2} cdot 2a cdot frac{1}{a} = 1$ unit.






share|cite|improve this answer











$endgroup$





















    1












    $begingroup$

    Hint:



    WLOG $b=1/a>0$



    $$xy-1implies xdfrac{dy}{dx}+y=0impliesdfrac{dy}{dx}_{text{ at }(a,b)}=-dfrac{1/a}a=-dfrac1{a^2}$$



    The equation of the tangent at $P,$ $$dfrac{y-1/a}{x-a}=-dfrac1{a^2}iff x+a^2y=2a$$



    As $x$ axis, $y=0,$ the intersection will be $A(2a,0)$



    $$|AP|=?, |OP|=?$$






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      The tangent line to $y=frac 1x$ at $P(x_0,y_0)$ is:
      $$y=y_0+y'(x_0)(x-x_0)=frac 1{x_0}-frac 1{x_0^2}(x-x_0).$$
      The point $A(x_1,0)$ has coordinate $x_1$:
      $$0=frac 1{x_0}-frac 1{x_0^2}(x_1-x_0) Rightarrow x_1=2x_0,$$
      which shows the triangle $PAO$ is isosceles, because the altitude $PB$ is the median too.



      Now the area of $Delta PAO$ is:
      $$S=frac 12 cdot AOcdot PB=frac12 cdot 2x_0 cdot frac1{x_0}=1.$$






      share|cite|improve this answer









      $endgroup$













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        3 Answers
        3






        active

        oldest

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        3 Answers
        3






        active

        oldest

        votes









        active

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        active

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        0












        $begingroup$

        To start approaching the problem, first notice that if P lies on the curve then $P (a, b) = P (a, frac{1}{a})$.



        Secondly, find the slope of the tangent line at $P$, which is precisely $-frac{1}{a^2}$



        Thus equation of the tangent line through $P$ is given by



        $$(y - frac{1}{a}) = (-frac{1}{a^2})(x-a) implies a^2y = 2a - x$$



        The above line intersects the $x$-axis only at $(2a, 0)$, thus $ A = (2a, 0)$.



        Therefore the coordinates of $Delta AOP$ are precisely: $$A (2a, 0), O(0, 0), P (a, frac{1}{a})$$
        making $AOP$ isosceles. Moreover, the area of $Delta AOP = frac{1}{2} cdot 2a cdot frac{1}{a} = 1$ unit.






        share|cite|improve this answer











        $endgroup$


















          0












          $begingroup$

          To start approaching the problem, first notice that if P lies on the curve then $P (a, b) = P (a, frac{1}{a})$.



          Secondly, find the slope of the tangent line at $P$, which is precisely $-frac{1}{a^2}$



          Thus equation of the tangent line through $P$ is given by



          $$(y - frac{1}{a}) = (-frac{1}{a^2})(x-a) implies a^2y = 2a - x$$



          The above line intersects the $x$-axis only at $(2a, 0)$, thus $ A = (2a, 0)$.



          Therefore the coordinates of $Delta AOP$ are precisely: $$A (2a, 0), O(0, 0), P (a, frac{1}{a})$$
          making $AOP$ isosceles. Moreover, the area of $Delta AOP = frac{1}{2} cdot 2a cdot frac{1}{a} = 1$ unit.






          share|cite|improve this answer











          $endgroup$
















            0












            0








            0





            $begingroup$

            To start approaching the problem, first notice that if P lies on the curve then $P (a, b) = P (a, frac{1}{a})$.



            Secondly, find the slope of the tangent line at $P$, which is precisely $-frac{1}{a^2}$



            Thus equation of the tangent line through $P$ is given by



            $$(y - frac{1}{a}) = (-frac{1}{a^2})(x-a) implies a^2y = 2a - x$$



            The above line intersects the $x$-axis only at $(2a, 0)$, thus $ A = (2a, 0)$.



            Therefore the coordinates of $Delta AOP$ are precisely: $$A (2a, 0), O(0, 0), P (a, frac{1}{a})$$
            making $AOP$ isosceles. Moreover, the area of $Delta AOP = frac{1}{2} cdot 2a cdot frac{1}{a} = 1$ unit.






            share|cite|improve this answer











            $endgroup$



            To start approaching the problem, first notice that if P lies on the curve then $P (a, b) = P (a, frac{1}{a})$.



            Secondly, find the slope of the tangent line at $P$, which is precisely $-frac{1}{a^2}$



            Thus equation of the tangent line through $P$ is given by



            $$(y - frac{1}{a}) = (-frac{1}{a^2})(x-a) implies a^2y = 2a - x$$



            The above line intersects the $x$-axis only at $(2a, 0)$, thus $ A = (2a, 0)$.



            Therefore the coordinates of $Delta AOP$ are precisely: $$A (2a, 0), O(0, 0), P (a, frac{1}{a})$$
            making $AOP$ isosceles. Moreover, the area of $Delta AOP = frac{1}{2} cdot 2a cdot frac{1}{a} = 1$ unit.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Dec 14 '18 at 13:54

























            answered Dec 13 '18 at 20:45









            thesagniksahathesagniksaha

            164115




            164115























                1












                $begingroup$

                Hint:



                WLOG $b=1/a>0$



                $$xy-1implies xdfrac{dy}{dx}+y=0impliesdfrac{dy}{dx}_{text{ at }(a,b)}=-dfrac{1/a}a=-dfrac1{a^2}$$



                The equation of the tangent at $P,$ $$dfrac{y-1/a}{x-a}=-dfrac1{a^2}iff x+a^2y=2a$$



                As $x$ axis, $y=0,$ the intersection will be $A(2a,0)$



                $$|AP|=?, |OP|=?$$






                share|cite|improve this answer









                $endgroup$


















                  1












                  $begingroup$

                  Hint:



                  WLOG $b=1/a>0$



                  $$xy-1implies xdfrac{dy}{dx}+y=0impliesdfrac{dy}{dx}_{text{ at }(a,b)}=-dfrac{1/a}a=-dfrac1{a^2}$$



                  The equation of the tangent at $P,$ $$dfrac{y-1/a}{x-a}=-dfrac1{a^2}iff x+a^2y=2a$$



                  As $x$ axis, $y=0,$ the intersection will be $A(2a,0)$



                  $$|AP|=?, |OP|=?$$






                  share|cite|improve this answer









                  $endgroup$
















                    1












                    1








                    1





                    $begingroup$

                    Hint:



                    WLOG $b=1/a>0$



                    $$xy-1implies xdfrac{dy}{dx}+y=0impliesdfrac{dy}{dx}_{text{ at }(a,b)}=-dfrac{1/a}a=-dfrac1{a^2}$$



                    The equation of the tangent at $P,$ $$dfrac{y-1/a}{x-a}=-dfrac1{a^2}iff x+a^2y=2a$$



                    As $x$ axis, $y=0,$ the intersection will be $A(2a,0)$



                    $$|AP|=?, |OP|=?$$






                    share|cite|improve this answer









                    $endgroup$



                    Hint:



                    WLOG $b=1/a>0$



                    $$xy-1implies xdfrac{dy}{dx}+y=0impliesdfrac{dy}{dx}_{text{ at }(a,b)}=-dfrac{1/a}a=-dfrac1{a^2}$$



                    The equation of the tangent at $P,$ $$dfrac{y-1/a}{x-a}=-dfrac1{a^2}iff x+a^2y=2a$$



                    As $x$ axis, $y=0,$ the intersection will be $A(2a,0)$



                    $$|AP|=?, |OP|=?$$







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 13 '18 at 14:22









                    lab bhattacharjeelab bhattacharjee

                    225k15157275




                    225k15157275























                        0












                        $begingroup$

                        The tangent line to $y=frac 1x$ at $P(x_0,y_0)$ is:
                        $$y=y_0+y'(x_0)(x-x_0)=frac 1{x_0}-frac 1{x_0^2}(x-x_0).$$
                        The point $A(x_1,0)$ has coordinate $x_1$:
                        $$0=frac 1{x_0}-frac 1{x_0^2}(x_1-x_0) Rightarrow x_1=2x_0,$$
                        which shows the triangle $PAO$ is isosceles, because the altitude $PB$ is the median too.



                        Now the area of $Delta PAO$ is:
                        $$S=frac 12 cdot AOcdot PB=frac12 cdot 2x_0 cdot frac1{x_0}=1.$$






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          The tangent line to $y=frac 1x$ at $P(x_0,y_0)$ is:
                          $$y=y_0+y'(x_0)(x-x_0)=frac 1{x_0}-frac 1{x_0^2}(x-x_0).$$
                          The point $A(x_1,0)$ has coordinate $x_1$:
                          $$0=frac 1{x_0}-frac 1{x_0^2}(x_1-x_0) Rightarrow x_1=2x_0,$$
                          which shows the triangle $PAO$ is isosceles, because the altitude $PB$ is the median too.



                          Now the area of $Delta PAO$ is:
                          $$S=frac 12 cdot AOcdot PB=frac12 cdot 2x_0 cdot frac1{x_0}=1.$$






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            The tangent line to $y=frac 1x$ at $P(x_0,y_0)$ is:
                            $$y=y_0+y'(x_0)(x-x_0)=frac 1{x_0}-frac 1{x_0^2}(x-x_0).$$
                            The point $A(x_1,0)$ has coordinate $x_1$:
                            $$0=frac 1{x_0}-frac 1{x_0^2}(x_1-x_0) Rightarrow x_1=2x_0,$$
                            which shows the triangle $PAO$ is isosceles, because the altitude $PB$ is the median too.



                            Now the area of $Delta PAO$ is:
                            $$S=frac 12 cdot AOcdot PB=frac12 cdot 2x_0 cdot frac1{x_0}=1.$$






                            share|cite|improve this answer









                            $endgroup$



                            The tangent line to $y=frac 1x$ at $P(x_0,y_0)$ is:
                            $$y=y_0+y'(x_0)(x-x_0)=frac 1{x_0}-frac 1{x_0^2}(x-x_0).$$
                            The point $A(x_1,0)$ has coordinate $x_1$:
                            $$0=frac 1{x_0}-frac 1{x_0^2}(x_1-x_0) Rightarrow x_1=2x_0,$$
                            which shows the triangle $PAO$ is isosceles, because the altitude $PB$ is the median too.



                            Now the area of $Delta PAO$ is:
                            $$S=frac 12 cdot AOcdot PB=frac12 cdot 2x_0 cdot frac1{x_0}=1.$$







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Dec 13 '18 at 16:24









                            farruhotafarruhota

                            20.2k2738




                            20.2k2738






























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