两个二项式相乘,在合并同类项前应该有几项?合并同类项以后,积可能会是三项吗?积可能是二项吗?请举出例子计算下面各题:(1)(a+5) (a-5) =__________ ;(2) (m+3)(m-3) =__________;(3) (3x+7)(3x-7) =__________; (4) (5a+b)(5a-b)=__________;(5)(n+3m)(n-3m)=__________,(6) (x+2y)(x-2y)=__________.通过计