How do you find a function f(x), which, when multiplied by its derivative, gives you x^3x3, and for which f(0) = 4f(0)=4?

1 Answer
Mar 1, 2015

The answer is: y=sqrt(x^4/2+16)y=x42+16.

This is a separable variable first-order differential equation:

yy'=x^3rArry(dy)/(dx)=x^3rArrydy=x^3dxrArr

intydy=intx^3dxrArry^2/2=x^4/4+crArry=+-sqrt(x^4/2+2c)

Since the initial condition is:

f(0)=4, we have to choose the positive one, and than we can find the constant c:

4=sqrt(0/2+2c)rArr16=2crArrc=8.

So the function is:

y=sqrt(x^4/2+16).