Elementary Separable Differential Equation with tricky integral The problem is to solve the separable differential equation sqrt(xy)~~(dy)/(dx)=sqrt(4-x)

Dorsheele0p

Dorsheele0p

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2022-08-19

Elementary Separable Differential Equation with tricky integral
Dear Plaimath. The problem is to solve the separable differential equation x y     d y d x = 4 x
My work thus far: x y     d y = 4 x     d x y     d y = 4 x x     d x y     d y = 4 x x     d x 2 3 y 3 / 2 = 4 x x     d x 2 3 y 3 / 2 = 4 x 1     d x
Is there an easier approach I have missed, or should I persist in trying to solve the integral.

Answer & Explanation

Chaya Garza

Chaya Garza

Beginner2022-08-20Added 10 answers

x y     d y d x = 4 x x y     d y = 4 x     d x y     d y = 4 x x     d x y     d y = 4 x x     d x 2 3 y 3 / 2 = 4 x x     d x  Make the substitution u = √x  2 3 y 3 / 2 = 2 4 u 2 d u  Substitute w = u/2  2 3 y 3 / 2 = 8 1 w 2   d w  using trig substitution 2 3 y 3 / 2 = 4 w 1 w 2 + 4 arcsin ( w ) + C back substituting  2 3 y 3 / 2 = 4 u 2 1 ( u 2 ) 2 + 4 arcsin ( u 2 ) + C 2 3 y 3 / 2 = 2 x 1 ( x 2 ) 2 + 4 arcsin ( x 2 ) + C 2 3 y 3 / 2 = 2 x 1 ( x 4 ) + 4 arcsin ( x 2 ) + C 2 3 y 3 / 2 = 2 x 4 x 4 + 4 arcsin ( x 2 ) + C 2 3 y 3 / 2 = x 4 x + 4 arcsin ( x 2 ) + C y 3 / 2 = 3 2 ( x 4 x + 4 arcsin ( x 2 ) + C ) y 3 / 2 = ( 3 2 x 4 x + 6 arcsin ( x 2 ) + 3 2 C ) y = ( 3 2 x 4 x + 6 arcsin ( x 2 ) + C ) 2 / 3
pleitatsj1

pleitatsj1

Beginner2022-08-21Added 4 answers

Make the substitution t = x , then
2 4 x   d x 2 x = 2 4 t 2   d t
This is a standard trig substitution. Let t = 2 sin θ
2 4 t 2 d t = 2 2 cos θ 2 cos θ   d θ = 8 cos 2 θ   d θ = 4 ( 1 + cos 2 θ )   d θ = 4 θ + 2 sin 2 θ + C
Going backwards
4 θ + 2 sin 2 θ + C = 4 θ + 4 sin θ cos θ + C = 4 arctan t 2 + t 4 t 2 + C = 4 arcsin x 2 + x 4 x + C

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