56*78,21*47怎樣簡便運算

題目：

56*78,21*47怎樣簡便運算

解答：

56x78
=(50+6)x(80-2)
=4000-100+480-12
=4368
21x47
=(20+1)x(50-3)
=1000-60+50-3
=987