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Q.

The coordinate of the point p(x,y)  lying in the first quadrant on the ellipse

x28+y218=1  so that the area of the triangle formed by the tangent at p and the coordinate axes is the smallest , are given by

 

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a

(2,3)

b

(8,0)

c

(18,0)

d

None of these

answer is A.

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Detailed Solution

Any point on the ellipse is given by (8cosθ,18sinθ)

Now      2x8+218ydydx=0dydx=9x4y

dydx|(8cosθ,18sinθ)=98cosθ418sinθ=92cotθ

Hence the equation of the tangent at (8cosθ,18sinθ)  is

y18sinθ=92cotθ(x8cosθ)

Therefore , the tangent cuts the coordinate axes at the points

(0,18sinθ)and(8cosθ,0)

Thus the area of the triangle formed by this tangent and the coordinate axes is

A=1218.8.1cosθsinθ

     =6cosθsinθ=12cosec2θ

But cosec2θ  is smallest when θ=π/4.

Therefore A is smallest when θ=π/4.

Hence the required point is (8.12,18.12)=(2,3)  

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