(sec x + tan x)^2
= (1 + sin x)^2/cos^2 x
= (1 + sin^2 x + 2 sin x ) /(1 - sin^2 x)
= (2 sin^2 x + cos^2 x + 2 sin x)/(1 - sin^2 x)
= 2 sin x(sin x + 1) + cos^2 x / (1 - sin^2 x)
= 2 sin x(sin x + 1) + 1 - sin^2 x / (1 - sin^2 x)
= (sin^2 x + 2 sin x + 1) / (1 - sin^2 x)
= (1 + sin x)(1 + sin x) / (1 + sin x)(1 - sin x)
= (1 + sin x) / (1 - sin x)