How do I find the integral of #f(x)=sec^5(x)#?

1 Answer
Mar 8, 2018

Answer:

#int sec^5xdx = (2sec^3xtanx + 3secxtanx + 3 ln abs(secx+tanx))/8 +C#

Explanation:

Note that in general for any #n >=1#:

#sec^(2n+1) x = sec^(2n-1)xsec^2x#

and:

#sec^2x = d/dx tanx#

so we can integrate by parts:

#int sec^(2n+1)xdx = int sec^(2n-1)d(tanx)#

#int sec^(2n+1)xdx = sec^(2n-1)xtanx - int tanx d(sec^(2n-1)x)#

#int sec^(2n+1)xdx = sec^(2n-1)xtanx - (2n-1)int tan^2xsec(2n-1)x#

Use now the trigonometric identity:

#tan^2x = sec^2x -1#

to get:

#int sec^(2n+1)xdx = sec^(2n-1)xtanx + (2n-1)int (1-sec^2x)sec^(2n-1)x#

and using the linearity of the integral:

#int sec^(2n+1)xdx = sec^(2n-1)xtanx + (2n-1)int sec^(2n-1)xdx- (2n-1) int sec^(2n+1)xdx#

the integral now appears on both sides and we can solve for it:

#2nint sec^(2n+1)xdx = sec^(2n-1)xtanx + (2n-1)int sec^(2n-1)xdx#

#int sec^(2n+1)xdx = (sec^(2n-1)xtanx)/(2n) + (2n-1)/(2n)int sec^(2n-1)xdx#

So, for #n=2#:

#int sec^5xdx = (sec^3xtanx)/4 + 3/4int sec^3xdx#

and for #n=1#

#int sec^3xdx = (secxtanx)/2 + 1/2int secxdx#

Now:

#int secx dx = int secx (secx+tanx)/(secx+tanx) dx#

#int secx dx = int (sec^2x+secxtanx)/(secx+tanx) dx#

#int secx dx = int (d(secx+tanx))/(secx+tanx) #

#int secx dx = ln abs(secx+tanx) +C#

Putting the partial results together:

#int sec^5xdx = (2sec^3xtanx + 3secxtanx + 3ln abs(secx+tanx))/8 +C#