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How to Integrate Sin^2 X

by Petra Wakefield, Demand Media

    The solution to the integral of sin^2(x) requires you to recall principles of both trigonometry and calculus. Don't conclude that since the integral of sin(x) equals -cos(x), the integral of sin^2(x) should equal -cos^2(x); in fact, the answer does not contain a cosine at all. You cannot directly integrate sin^2(x). Use trigonometric identities and calculus substitution rules to solve the problem.

    Step 1

    Use the half angle formula, sin^2(x) = 1/2*(1 - cos(2x)) and substitute into the integral so it becomes 1/2 times the integral of (1 - cos(2x)) dx.

    Step 2

    Set u = 2x and du = 2dx to perform u substitution on the integral. Since dx = du/2, the result is 1/4 times the integral of (1 - cos(u)) du.

    Step 3

    Integrate the equation. Since the integral of 1du is u, and the integral of cos(u) du is sin(u), the result is 1/4*(u - sin(u)) + c.

    Step 4

    Substitute u back into the equation to get 1/4*(2x - sin(2x)) + c. Simplify to get x/2 - (sin(x))/4 + c.

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    • For a definite integral, eliminate the constant in the answer and evaluate the answer over the interval specified in the problem. If the interval is 0 to 1, for example, evaluate [1/2 - sin(1)/4] - [0/2 - sin(0)/4)].

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