Rút gọn: A = sin^4(a) - cos^4(a) + 2cos^2(a)

Rút gọn: 
A=sin^4(a)-cos^4(a)+2cos^2(a)
B=(1-sin^2(a)).cot^2(a)+1-cot^2(a)
C=sin^2(10o)+sin^2(20o)+...+sin^2(80o)