Monday, July 8, 2013

What is the antiderivative of the function f(x) = sin^3 x?

We'll calculate the antiderivative F(x) integrating the
given function.


F(x) is a function such as dF/dx =
f(x).


Int f(x)dx = F(x) +
C


We'll write the function as a
product:


(sinx)^3 = (sinx)^2*sin
x


We'll integrate both
sides:


Int (sinx)^3dx = Int [(sinx)^2*sin
x]dx


We'll write (sinx)^2  = 1 -
(cosx)^2


Int [(sinx)^2*sin x]dx = Int [(1 - (cosx)^2)*sin
x]dx


We'll remove the
brackets:


Int [(1 - (cosx)^2)*sin x]dx  = Int sin xdx - Int
(cosx)^2*sin xdx


We'll solve Int (cosx)^2*sin xdx using
substitution technique:


cos x =
t


We'll differentiate both
sides:


cos xdx = dt


We'll
re-write the integral, changing the variable:


Int
(cosx)^2*sin xdx = Int t^2dt


Int t^2dt = t^3/3 +
C


Int (cosx)^2*sin xdx = (cos x)^3/3 +
C


Int (sinx)^3dx = Int sin xdx - Int (cosx)^2*sin
xdx


Int (sinx)^3dx = -cos x - (cos x)^3/3 +
C

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