What is f(x) = int 3x^3-2x+xe^(x-2) dxf(x)=3x32x+xex2dx if f(1) = 3 f(1)=3?

1 Answer
Mar 27, 2017

f(x)=3x^4/4 -2x^2/2+(x-1)e^(x-2)+13/4f(x)=3x442x22+(x1)ex2+134

Explanation:

Let's first evaluate the indefinite integral:

f(x)=int(3x^3-2x+xe^(x-2))dxf(x)=(3x32x+xex2)dx

=int(3x^3)dx -int(2x)dx+int(xe^(x-2))dx=(3x3)dx(2x)dx+(xex2)dx

=3x^4/4 -2x^2/2+C+int(xe^(x-2))dx=3x442x22+C+(xex2)dx where CC is the constant of integration.

We can evaluate int(xe^(x-2))dx(xex2)dx using integration by parts as follows:

int(xe^(x-2))dx = e^-2int(xe^(x))dx(xex2)dx=e2(xex)dx

Evaluating int(xe^(x))dx(xex)dx:
int(xe^(x-2))dx = xe^x - int(e^x)dx(xex2)dx=xex(ex)dx
= xe^x - e^x +C=xexex+C

Therefore, int(xe^(x-2))dx = e^-2(xe^x - e^x)+C(xex2)dx=e2(xexex)+C
=(x-1)e^(x-2)+C=(x1)ex2+C

Therefore, the original integral is equal to:

f(x)=3x^4/4 -2x^2/2+(x-1)e^(x-2)+Cf(x)=3x442x22+(x1)ex2+C

Plugging in x=1x=1, we get:

f(1)=3/4 -1+0+C =-1/4+Cf(1)=341+0+C=14+C

But f(1)=3f(1)=3

So, 3=-1/4+C => C=3+1/4=13/43=14+CC=3+14=134

Therefore,

f(x)=3x^4/4 -2x^2/2+(x-1)e^(x-2)+13/4f(x)=3x442x22+(x1)ex2+134