Let P(x,a) and Q(-x,a) be two points on the upper half of the ellipse(adsbygoogle = window.adsbygoogle || []).push({});

[tex] \frac{x^2}{100}+\frac{(y-5)^2}{25}=1 [/tex]

centered at (0,5). A triangle RST is formed by using the tangent lines to the ellipse at Q and P.

Show that the area of the triangle is

[tex]A(x)=-f'(x)[x-\frac{f(x)}{f'(x)}]^2 [/tex]

where y=f(x) is the function representing the upper half of the ellipse.

I know f(x) and f'(x). I just cannot get A(x). I'm going mad please help.

Thanks in advance

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# Circumscribing an ellipse

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