How do you integrate int 1/sqrt(3x-12sqrtx+29) using trigonometric substitution?

1 Answer
Jul 16, 2018

=2/3(sqrt(3x-12sqrt(x)+29)+2sqrt(3) sinh^-1(sqrt(3/17)(sqrt(x)-2)))+C

Explanation:

For the integrand 1/(sqrt(3x-12sqrt(x)+29)), substitute

u=sqrt(x) and du=1/(2sqrt(x))dx

=2int(u/(sqrt(3u^2-12u+29)))du

Completing the square, we get

=2int(u/(sqrt((sqrt(3)u-2sqrt(3))^2+17)))du

Let s=sqrt(3)u-2sqrt(3) and ds=sqrt(3)du

=2/sqrt(3) int ((s+2sqrt(3))/(sqrt(3)sqrt(s^2+17))ds)

Factor out the constant 1/sqrt(3)

=2/3 int(s+2sqrt(3))/(sqrt(s^2+17)) ds

Next, substitute s=sqrt(17) tan(p) and ds=sqrt(17)sec^2(p)dp

Then, sqrt(s^2+17)=sqrt(17tan^2(p)+17)=sqrt(17)sec(p) and p=tan^-1(s/sqrt(17)). This gives us

=(2sqrt(17))/3 int (sqrt(17)tan(p)+2sqrt(3)sec(p))/sqrt(17) dp

Factoring out 1/sqrt(17) gives

=2/3 int (sqrt(17)tan(p) + 2sqrt(3))sec(p) dp

=2/3 int (sqrt(17)tan(p)sec(p) + 2sqrt(3)sec(p)) dp

=(2sqrt(17))/3 int (sqrt(17)tan(p)sec(p))dp + 4/sqrt(3) int sec(p)) dp

Substitute w=sec(p) and dw=tan(p)sec(p)dp

=(2sqrt(17))/3 int 1dw + 4/sqrt(3) intsec(p)dp

=(2sqrt(17)w)/3+4/sqrt(3) int sec(p)dp

=(4 ln(tan(p)+sec(p)))/sqrt(3) + (2sqrt(17)3)/3+C

Backwards substituting all the variables eventually gives

=2/3(sqrt(3x-12sqrt(x)+29)+2sqrt(3) sinh^-1(sqrt(3/17)(sqrt(x)-2)))+C