Evaluate: `intsec^4x\ dx`

Explanation

It is given that `int sec^4 x dx`
We can write it as

`= int sec^2 x sec^2 x dx`
So we get `= int (1 + tan^2 x) sec^2 x dx`

Take `tan x = t`

By differentiation we get `sec^2 x dx = dt`

It can be written as

`= int(1 + t^2) dt`

By integrating w.r.t. `t`

`= t + t^3/3 + c`
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