Math Archive (9)

Calculus Level 5

f ( x ) = x + 0 1 ( x 2 y + y 2 x ) f ( y ) d y {f(x)=x+\int_{0}^{1}(x^{2}y+y^{2}x)f(y) \, dy} f ( x ) = x + 61 x + 80 x 2 ω {f(x)=x+\dfrac{61x+80x^{2}}{\omega}} Find ω 2 \omega-2 .

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The answer is 117.

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1 solution

f ( x ) = x + 0 1 ( x 2 y + y 2 x ) f ( y ) d y \displaystyle f\left( x \right) =x+\int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }y+{ y }^{ 2 }x \right) f\left( y \right) dy }

= x + 0 1 ( x 2 y ) f ( y ) d y + 0 1 y 2 x f ( y ) d y \displaystyle =x+\int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }y \right) f\left( y \right) dy } +\int _{ 0 }^{ 1 }{ { y }^{ 2 }xf\left( y \right) dy }

= x + A x 2 + B x \displaystyle =x+A{ x }^{ 2 }+Bx where A = 0 1 ( y ) f ( y ) d y , B = 0 1 y 2 f ( y ) d y \displaystyle A=\int _{ 0 }^{ 1 }{ \left( y \right) f\left( y \right) dy } , B=\int _{ 0 }^{ 1 }{ { y }^{ 2 }f\left( y \right) dy }

0 1 x f ( x ) d x = 0 1 x ( x + A x 2 + B x ) d x = A 4 + B + 1 3 = A \displaystyle \int _{ 0 }^{ 1 }{ xf\left( x \right) dx } =\int _{ 0 }^{ 1 }{ x\left( x+A{ x }^{ 2 }+Bx \right) dx } =\frac { A }{ 4 } +\frac { B+1 }{ 3 } =A

0 1 x 2 f ( x ) d x = 0 1 x 2 ( x + A x 2 + B x ) d x = A 5 + B + 1 4 = B \displaystyle \int _{ 0 }^{ 1 }{ { x }^{ 2 }f\left( x \right) dx } =\int _{ 0 }^{ 1 }{ { x }^{ 2 }\left( x+A{ x }^{ 2 }+Bx \right) dx } =\frac { A }{ 5 } +\frac { B+1 }{ 4 } =B

Solve simultaneous equations and we get,

A = 80 119 , B = 61 119 \displaystyle A=\frac { 80 }{ 119 } , B=\frac { 61 }{ 119 }

Hence,

ω = 119 \displaystyle \omega=119

ω 2 = 117 \displaystyle \omega-2=\boxed{117}

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