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(tgα*ctgα=1
tg45=1)
b)ctg(90-65)*ctg(90-55)*ctg45*ctg55*ctg65=tg65*tg55*ctg45*ctg55*ctg65=1*1*1=1
c)sin²(90-15)+sin²15-2*1/2=cos²15+sin²15-1=1-1=0
d) cos²52-2*cos52*cos38+cos²38+2*sin38*sin52+2*cos60=cos²(90-38)-2cos(90-38)cos(90-52)+cos²38+2sin38*sin52+2cos60=sin²38-2sin38sin52+cos²38+2sin38sin52+2*1/2=1+1=2