Area Question 37

Question 37

  1. Let C1 and C2 be the graphs of functions y=x2 and y=2x,0x1, respectively. Let C3 be the graph of a function y=f(x),0x1,f(0)=0. For a point P on C1, let the lines through P, parallel to the axes, meet C2 and C3 at Q and R respectively (see figure). If for every position of P( on C1) the areas of the shaded regions OPQ and ORP are equal, then determine f(x).

(1998,8M)

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Solution:

  1. Refer to the figure given in the question. Let the coordinates of P be (x,x2), where 0x1.

For the area (OPRO),

Upper boundary: y=x2 and

lower boundary : y=f(x)

Lower limit of x:0

Upper limit of x:x

Area (OPRO)=0xt2dt0xf(t)dt

=t33x0xf(t)dt =x330xf(t)dt

For the area (OPQO),

The upper curve : x=y

and the lower curve : x=y/2

Lower limit of y:0

and upper limit of y:x2

$$ \begin{aligned} \therefore \text { Area }(O P Q O) & =\int_{0}^{x^{2}} \sqrt{t} d t-\int_{0}^{x^{2}} \frac{t}{2} d t \ & =\frac{2}{3}\left[t^{3 / 2}\right]{0}^{x^{2}}-\frac{1}{4}\left[t^{2}\right]{0}^{x^{2}} \ & =\frac{2}{3} x^{3}-\frac{x^{4}}{4} \end{aligned} $$

According to the given condition,

x330xf(t)dt=23x3x44

On differentiating both sides w.r.t. x, we get

x2f(x)1=2x2x3

f(x)=x3x2,0x1



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