(1)令t=tan (θ/2)由万能公式sinθ=(2t)/(t^2+1) cosθ=(t^2-1)/(t^2+1)原式=(t^2+1+2t-t^2+1)/(t^2+1+2t+t^2-1)=(2t+2)/(2t^2+2t)=1/t=cot (θ/2)(2)令t=tan(3a/2) tan 3a=(2t)/(1-t^2)原式=t*(2t)/(1-t^2)+1=(...???