What is f(x) = int xe^x-x dx if f(-1) = 1 ?

1 Answer
Jul 30, 2018

f(x) = 2/e+ e^x(x-1)+(3-x^2)/2

Explanation:

Using the fundamental theorem of calculus:

f(x) = 1+ int_(-1)^x (te^t-t)dt

as the integral is linear:

f(x) = 1+ int_(-1)^x te^tdt- int_(-1)^x tdt

Solve the two integrals separately:

int_(-1)^x tdt = [t^2/2]_(-1)^x

int_(-1)^x tdt = x^2/2-1/2

and:

int_(-1)^x te^tdt = int_(-1)^x t d/dt(e^t) dt

integrating by parts:

int_(-1)^x te^tdt = - int_(-1)^x e^t dt + [te^t]_( -1)^x

int_(-1)^x te^tdt = - [e^t]_(-1)^x + xe^x+1/e

int_(-1)^x te^tdt = - e^x+1/e + xe^x+1/e

int_(-1)^x te^tdt = e^x(x-1)+2/e

Putting together the partial results:

f(x) = 1+ e^x(x-1)+2/e-x^2/2+1/2

f(x) = 2/e+ e^x(x-1)+(3-x^2)/2