Differential equation!

Calculus Level 3

y ( x ) y(x) satisfies:

( 2 x y + x 2 y + y 3 3 ) d x + ( x 2 + y 2 ) d y = 0 \large \left(2xy+x^2y+\frac {y^3}{3}\right)dx+\left(x^2+y^2\right)dy=0

If y ( 1 ) = 1 y(1)=1 , find the value of ( y ( 0 ) ) 3 (y(0))^3 .


The answer is 10.873.

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1 solution

Parth Sankhe
Nov 30, 2018

( 2 x y d x + x 2 d y ) + x 2 y d x + ( y 3 3 d x + y 2 d y ) = 0 (2xydx+x^2dy) + x^2ydx + (\frac {y^3}{3}dx + y^2dy)=0

Putting x 2 y = t x^2y=t and y 3 3 = u \frac {y^3}{3} = u

( d t + t d x ) + ( u d x + d u ) = 0 (dt+tdx)+(udx+du)=0

Presence of f ( x ) + f ( x ) f(x) + f'(x) highly suggests to multiply both sides by e x e^x

( e x d t + t e x d x ) + ( u e x d x + e x d u ) = 0 (e^xdt + t\cdot e^xdx)+ (u\cdot e^xdx+e^xdu)=0

, d ( e x t ) + d ( e x u ) = 0 \therefore, d(e^xt) + d(e^xu)=0

e x ( t + u ) + c = 0 e^x(t+u)+c=0 , where c c is the constant is integration.

Put in the values of t t and u u , use y ( 1 ) = 1 y(1)=1 to find c c , you should get c = 4 e 3 c=\frac {4e}{3} , and the value of y 3 ( 0 ) y^3(0) should come out to be 4 e \large 4e .

Thanks for the solution :)

A Former Brilliant Member - 2 years, 6 months ago

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