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Question 56 of 62

Q.Find the area of region between parabolas y2=4axy^2 = 4ax and x2=4ayx^2 = 4ay.

Maharashtra MsbshseMaharashtra HSC (MSBSHSE) Board 2022Subjective· 4mImportance★★★★★
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Find the intersection points, then integrate the difference of the two curves.

y2=4axy^2=4ax and x2=4ayx^2=4ay. From the second, y=x24ay=\dfrac{x^2}{4a}; substituting into the first:

(x24a)2=4ax  ⟹  x416a2=4ax  ⟹  x4=64a3x  ⟹  x(x3−64a3)=0\left(\dfrac{x^2}{4a}\right)^2=4ax \implies \dfrac{x^4}{16a^2}=4ax \implies x^4=64a^3x \implies x(x^3-64a^3)=0

So x=0x=0 or x=4ax=4a, giving intersection points (0,0)(0,0) and (4a,4a)(4a,4a).

For 0≤x≤4a0\le x\le4a, y=2axy=2\sqrt{ax} (upper branch of y2=4axy^2=4ax) lies above y=x24ay=\dfrac{x^2}{4a} (checked e.g. at x=ax=a: 2a>a/42a > a/4).

Area=∫04a(2ax−x24a)dx=2a[x3/23/2]04a−14a[x33]04a\text{Area} = \int_0^{4a}\left(2\sqrt{ax}-\dfrac{x^2}{4a}\right)dx = 2\sqrt a\left[\dfrac{x^{3/2}}{3/2}\right]_0^{4a} - \dfrac1{4a}\left[\dfrac{x^3}3\right]_0^{4a}

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