Computing \(\displaystyle{\int_{{0}}^{{{2}\pi}}}{\left({1}+{2}{\cos{{t}}}\right)}^{{n}}{\cos{{n}}}{t}{\left.{d}{t}\right.}\)

talpajocotefnf3

talpajocotefnf3

Answered question

2022-03-28

Computing 02π(1+2cost)ncosntdt

Answer & Explanation

kachnaemra

kachnaemra

Beginner2022-03-29Added 16 answers

It's standard to use the following substitution in these cases:
z=eit, dt=dziz
cost=12(z+1z), e=zn
Therefore if C(0,1)=C is the unit circle centered at the origin, we have
I=02π(1+2cost)ncosntdt=Re
02π(1+2cost)nedt=C(1+z+1z)nzndziz
and we now get a simplification as the singularity at the origin is reduced to a simple pole:
I=ReC(z2+z+1)nizdz=Re(i2π1i)=2π
by Residue Theorem.

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