:(1-cost)(1+cost)= sin^2t+2cos^2t-1=

1)=1-cos^2 t=sin^2 t;
2)=sin^2 t+2 cos^2 t- sin^2t-cos^2t=cos^2t

Парочка формул:
cos²x+sin²x=1
2cos²x-1=cos²x-sin²x(=cos2x)

1) (1-cost)(1+cost)=1²-cos²t=sin²t

2) sin²t+(2cos²t-1)=sin²t+cos²t-sin²t=cos²t