(cosa sinp)²=(1-sin²a)(1-cos²p)
=(1-(sin²a+cos²p)+in²acos²p)
=(5/4-(sin²a+cos²p))
≤(5/4-2√(sin²acos²p)
=(5/4-1)
=1/4
即(cosa sinp)²≤1/4
-1/2≤cosa sinp≤1/2