What is the integral of cos^5 x?  

2 Answers

justaguide's profile pic

justaguide | College Teacher | (Level 2) Distinguished Educator

Posted on

We have to find the integral of f(x) = (cos x)^5.

Int [f(x)] = Int [(cos x)^4 cos x dx]

=> Int [(1 – (sin x)^2)^2 cos x dx]

let t = sin x

=> dt/dx = cos x,

=> dt = cos x dx.

Int [(1 – (sin x)^2)^2 cos x dx]

=> Int [(1 – t^2)^2 dt]

=> Int [1 + t^4 – 2t^2 dt]

=> Int [1 dt] – Int [2t^2] + Int [t^4]

=> t – (2/3)*t^3 + t^5/5 +C

replace t with sin x

=> sin x – (2/3)*(sin x)^3 + (sin x) ^5 /5 + C

Therefore the required integral of cos^5 x is sin x – (2/3)*(sin x)^3 + (sin x)^5 /5 + C.

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thomasterrific | Student, Undergraduate | eNotes Newbie

Posted on

Hey dudes, I just needed to use this integral for a calculation and used the same substitution. The answer I got was very similar: sinx - (2/3)sin^3x - (1/5)sin^5x So I have a negative (1/5)sin^5x term instead of positive, as was given in justaguide's solution. I could be / am probably wrong, but when I differentiated my answer, it gives 'cos^5x' (ie the integrand) Anybody else out there want to double-check? As I said, I'm quite likely to be wrong but, just in case I'm not, thought I'd bring it up so that people aren't using a potentially incorrect answer for assignments and whatnot. Alright, cheers.