他是我的朋友。他很瘦。可以说:
A:He is my friend. He's fat.
B:He is my friend. He's thin.
(Ⅰ)求f( 4π3 )的值;
(Ⅱ)若角A是△ABC的内角,且f(A)= 34 ,求cos2A﹣sin2A的值.