Evaluate : `intcosxcos2xcos3xdx`.

Explanation

`intcosxcos2xcos3xdx`
`=(1)/(2)int(2cosxcos2x)cos3xdx`
`=(1)/(2)int(cos3x+cosx)3xdx=(1)/(2)int(cos^(2)3x+cosxcos3x)dx`
`=(1)/(4)int(2cos^(2)3x)dx+(1)/(4)int(2cosxcos3x)dx`
`=(1)/(4)int(1+cos6x)dx+(1)/(4)int(cos4x+cos2x)dx`
`=(1)/(4)int(1+cos6x)dx+(1)/(4)intcos4x+(1)/(4)intcos2xdx`
`=(1)/(4)x+(1)/(4)*(sin6x)/(6)+(1)/(4)*(sin4x)/(4)+(1)/(4)*(sin2x)/(2)+C`
`=(x)/(4)+(sin6x)/(24)+(sin4x)/(16)+(sin2x)/(8)+C`.