B
banhbaocua1


CMR với mọi góc anppa (a) <45 luôn có:
a)sin 2 a = 2sin a . cos a
b)cos 2 a = cos^2 a - sin ^2 a = 2 cos ^2 a -1
c)cotg 2 a = (1- tg^2 a ) / (2 tg a )
a)sin 2 a = 2sin a . cos a
b)cos 2 a = cos^2 a - sin ^2 a = 2 cos ^2 a -1
c)cotg 2 a = (1- tg^2 a ) / (2 tg a )