Thursday, June 25, 2015

what is the antiderivative of (-2x+5)^6

We are asked to find an antiderivative of (-2x+5)^6 :
This is essentially equivalent to evaluating the indefinite integral int (-2x+5)^6
Now int u^n du=1/(n+1) u^(n+1) with u a differentiable function of x. Here we can use u-substitution with u=-2x+5 and du=-2dx. Multiplying the integrand by -2 and (-1/2) (thus multiplying by 1) we get:
int (-1/2)(-2)(-2x+5)^6 dx or -1/2 int (-2x+5)^6(-2dx)
Integrating we get (-1/2)(1/7)(-2x+5)^7+C where C is some real constant.
Thus an antiderivative for (-2x+5)^6 is -1/14 (-2x+5)^7 .
** We can check by taking the derivative, using the chain rule to get:
d/(dx)[ -1/14(-2x+5)^7]=(7)(-1/14)(-2x+5)^6(-2)=(-2x+5)^6 as required.
** Note that we find an antiderivative, as a whole family of functions exist whose derivative is the given function. We can add any constant to the given antiderivative to get another antiderivative.
** An alternative to integrating is to use guess and revise. We might guess that the function we seek is (-2x+5)^7 . Upon checking by taking the derivative, we note that we are off by a factor of -14 (since the derivative is 7(-2x+5)^6(-2)=-14(-2x+5)^6 ), so we introduce the factor -1/14 to compensate.
http://mathworld.wolfram.com/IndefiniteIntegral.html

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