Evaluate: `int1/(cos(x-a)cos(x-b))\ dx`

Explanation

`int(1)/(sin(x-a)sin(x-b))dx`
`=(1)/(sin(a-b))int(sin{(x-b)-(x-a)})/(sin(x-a)sin(x-b))dx`
`=(1)/(sin(a-b))xxint(sin(x-b)cos(x-a)-cos(x-b)sin(x-a))/(sin(x-a)sin(x-b))dx`
`=(1)/(sin(a-b))int{cot(x-a)-cot(x-b)}dx`
`=(1)/(sin(a-b)){log|sin(x-a)|-log|sin(x-b)|}+c`
`=("cosec"(a-b))log|(sin(x-a))/(sin(x-b))|+c`