>>286
t=2xとおけば
x->0 のとき t->0 だから
(与式の左辺) = lim_[t->0] (1+t)^(1/t) = e = (与式の右辺)
よって与式は成り立つ

>>281
sin20° = cos(90-20)° = cos70°
(sin(x))^2 + (cos(x))^2 = 1 より
(与式) = (cos70°)^2 + (sin70°)^2 = 1