JEE MAIN - Mathematics (2020 - 8th January Morning Slot - No. 12)

For a > 0, let the curves C1 : y2 = ax and C2 : x2 = ay intersect at origin O and a point P. Let the line x = b (0 < b < a) intersect the chord OP and the x-axis at points Q and R, respectively. If the line x = b bisects the area bounded by the curves, C1 and C2, and the area of
$$\Delta $$OQR = $${1 \over 2}$$, then 'a' satisfies the equation :
x6 – 12x3 + 4 = 0
x6 – 12x3 – 4 = 0
x6 + 6x3 – 4 = 0
x6 – 6x3 + 4 = 0

Explanation

JEE Main 2020 (Online) 8th January Morning Slot Mathematics - Area Under The Curves Question 106 English Explanation C1 : y2 = ax, C2 : x2 = ay (a > 0)

P : intersection point of y2 = ax and x2 = ay

$$ \Rightarrow $$ x4 = a2 y2

$$ \Rightarrow $$ x4 = a2ax

$$ \Rightarrow $$ x = a, y = a

$$ \therefore $$ Point P : (a, a)

Line OP : y = x

$$ \Rightarrow $$ Point Q = (b, b)

Area $$\Delta $$OQR = $${1 \over 2}$$

$$ \Rightarrow $$ $${1 \over 2} \times b \times b$$ = $${1 \over 2}$$

$$ \Rightarrow $$ b = 1

As line x = b bisect the area between curve

$$ \therefore $$ $${1 \over 2}\int\limits_0^a {\left( {\sqrt {ax} - {{{x^2}} \over a}} \right)} dx$$ = $$\int\limits_0^1 {\left( {\sqrt {ax} - {{{x^2}} \over a}} \right)} dx$$

$$ \Rightarrow $$ $$\left[ {{1 \over 2}\sqrt a {x^{{3 \over 2}}} \times {2 \over 3} - {1 \over 2}{{{x^3}} \over {3a}}} \right]_0^a$$ = $$\left[ {\sqrt a {x^{{3 \over 2}}} \times {2 \over 3} - {{{x^3}} \over {3a}}} \right]_0^1$$

$$ \Rightarrow $$ $${{{a^2}} \over 3} - {1 \over 2}{{{a^2}} \over 3}$$ - 0 = $${{2\sqrt a } \over 3} - {1 \over {3a}}$$ - 0

$$ \Rightarrow $$ $${{{a^2}} \over 2} + {1 \over a} = 2\sqrt a $$

$$ \Rightarrow $$ $${a^3} + 2 = 4a\sqrt a $$

Squareing both sides, we get

$$ \Rightarrow $$ $${a^6} + 4{a^3} + 4 = 16{a^3}$$

$$ \Rightarrow $$ $${a^6} - 12{a^3} + 4 = $$ 0

Hence $$a$$ satisfy x6 – 12x3 + 4 = 0.

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