Need help

[hoary mason]

u = x

dv = cos3x

why? because the uv term is trivial

but the - int vdu term is what you have to worry about

if we did u = cos3x and dv = x

we'd get du = -3sin3x and v = x^2/2

this makes the - int vdu term messy

but u = x and dv = cos3x gives us

du = 1 and v = sin3x/3

which is just a normal trigonometric integral

[storm crest]

Next time, please try asking OP first what they tried and work off of that, rather than setting everything up for them right away

[hoary mason]

ok

[analog inlet]

u=3x

that simple tbh