原式=[sin²(a/2)+cos²(a/2)-2sin(a/2)cos(a/2)]/[cos²(a/2)-sin²(a/2)]=[cos(a/2)-sin(a/2)]²/[cos(a/2)+sin(a/2)][cos(a/2)-sin(a/2)]=[cos(a/2)-sin(a/2)]/[cos(a/2)+sin(a/2)]分子分母...