A calculus problem by Akshay Yadav

Calculus Level 5

Find the area of given shaded part if the equation of curve is y = x 3 x + 1 y=-|x^3|-x+1 and the equation of line is y = x y=x .

If it can be written as a b + c c d \dfrac{a}{b}+\dfrac{c\sqrt{c}}{d} . Where ( a , b ) (a,b) and ( c , d ) (c,d) are pair of mutually prime positive integers and c c is square root free.

Then find a + b + c + d a+b+c+d .

Clarifications: The area you have to find is bounded by 0.4 0.4 on the right.


The answer is 11806.

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1 solution

Rishabh Jain
Mar 16, 2016

It's a delight to do calculations of this question without a calculator :-).....
First let us find the point of intersection of line and curve in third quadrant which is given by: x 3 x + 1 = x x 3 2 x + 1 = 0 ( x 1 ) ( x 2 + x 1 ) = 0 x = 1 5 2 = ϕ \begin{aligned}~~&x^3-x+1=x\\&\implies x^3-2x+1=0\\&\implies(x-1)(x^2+x-1)=0\\&\implies x=\dfrac{-1-\sqrt 5}{2}=-\phi\end{aligned} ( x < 0 ) (\because ~x<0) Area of shaded part is given by:- T = ϕ 0 ( x 3 2 x + 1 ) d x + 0 0.4 ( x 3 2 x + 1 ) d x \mathfrak{T}=\int_{-\phi}^0\left(x^3-2x+1\right)\, dx+\int_{0}^{0.4}\left(-x^3-2x+1\right)\, dx = ( x 4 4 x 2 + x ) ϕ 0 + ( x 4 4 x 2 + x ) 0 2 5 \large=\left(\dfrac{x^4}{4}-x^2+x\right)|_{-\phi}^0+\left(\dfrac{-x^4}{4}-x^2+x\right)|_{0}^{\frac{2}{5}} = ( ϕ 2 ϕ ϕ 4 4 ) + 146 625 =\left(\phi^2-\phi-\frac{\phi^4}{4}\right)+\dfrac{146}{625} Putting ϕ = 1 + 5 2 \phi=\dfrac{1+\sqrt 5}{2} and simplification gives:- T = 6793 5000 + 5 5 8 \Large\mathfrak{T}=\dfrac{6793}{5000}+\dfrac{5\sqrt5}{8} 6793 + 5000 + 5 + 8 = 11806 \therefore 6793+5000+5+8=\Huge\color{#456461}{\boxed{\color{#EC7300}{\boxed{\color{#0C6AC7}{\mathbf{11806}}}}}}

Note:-I have used everywhere the fact that x 3 = x 3 |x^3|=x^3 when x > 0 x>0 and x 3 = x 3 |x^3|=-x^3 when x < 0 x<0 .

Nice solution!

Akshay Yadav - 5 years, 3 months ago

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