cosa=cos[2(a/2)]/1={[cos(a/2)]^2-[sin(a/2)]^2]/{[cos(a/2)]^2+[sin(a/2)]^2}【分子、分母同除[cos(a/2)]^2得】={1-[tan(a/2)]^2}/{1+[tan(a/2)]}=(1-2^2)/(1+2^2)=-3/5.
求采纳