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Max area for triangle in ellipse...

This is the question :

there is an ellipse of the form

x^2/a^2 + y^2/b^2 = 1

find max area of isosceles triangle inscribed in the ellipse with its vertex at one end of major axis..

I uploaded the figure...

Now this is what I did.

From the figure, I took the semi height as y and the length of base as x+a so the area will be A = y*(x+a)

using the ellipse equation I got expression for y in terms of x and the constants a and b. then I did d/dx (A) = 0 to find the value of x, I got x = -a and x = 2a-1

so that means the length is 2a + 1 and y = (b/a ) * (SQRT(a^2-x^2))

and I get area as (b/a)((2a-1)^3/2) but the answer is b/a((3)^3/2)/4 ...

I want to know whats wrong, I dont want to know any new method because I know one method mentioned in the book that is to take it in parametric form ie x=acost and y = bsint

I want to know whats wrong with my method..

Thanks