如果多项式与多项式相等,那么 .
计算:51×49=_________.
(1)(3a-4b)( )=9a2-16b2; (2)(4+2x)( )=16-4x2;(3)(-7-x)( )=49-x2; (4)(-a-3b)(-3b+a)=_______.
计算:(1)(a+1)(a-1)=_________;(3)(-a+1)(a+1)=________;(3)(-a+1)(a+1)=________;(4)(a+1)(-a-1)=_______.
用字母表示平方差公式为:___________.
计算:(3x-y)(x+2y)=________.