Evaluate: limx→π/2 (secx - tanx)
Put y = π/2 - x. Then y→0 as x → π/2
limx→π/2 (secx - tanx) = limy→0 [ sec(π/2 - y) - tan(π/2 - y)]
= limy→0 (cscy - coty)
= limy→0 [(1/sin y) - (cos y/sin y)]
= limy→0 [(1 - cos y)/sin y]
= limy→0 [(2 sin2(y/2)) / (2 sin(y/2) cos(y/2))]
= limy→0 tan(y/2) = 0