### Author Topic: non-homogeneous guessing question  (Read 729 times)

#### Weina Zhu

• Jr. Member
• Posts: 6
• Karma: 1
##### non-homogeneous guessing question
« on: October 08, 2018, 10:37:22 AM »
during solving the general solution for the non-homogeneous equation, why should we guessing an answer combine sin&cos together for
an equation like the right-hand side only contains one of the sin&cos.

Thanks!

#### Victor Ivrii

Because derivative of sine is cosine and v.v. Try to solve $y''+y'=\cos(x)$ as with $y=a\cos(x)$.