`tan alpha = sin alpha / cos alpha`
`tan alpha = 1/text(cot α)`
`tan alpha = - cot(alpha + pi/2) = - cot(alpha - pi/2)`
`tan alpha = sqrt(1/cos^2alpha - 1)`
`-tan alpha = tan(-alpha)`
`(tan alpha)^' =1 /cos^2 alpha`
`int tan(alpha) dalpha = -ln |cos alpha| + C`