31乘29 9.9乘10.1 98乘102 九分之11乘九分之七 用平方差公式计算这是4到题 别看错了

31*29
=(30+1)(30-1)
=30^2-1^2
=899
9.9*10.1
=(10-0.1)(10+0.1)
=10^2-0.1^2
=99.9
98*102
=(100-2)(100+2)
=100^2-2^2
=9996
11/9*7/9
=(9/9+2/9)(9/9-2/9)
=(9/9)^2-(2/9)^2
=1-4/81
=77/81

31x29
=(30+1)(30-1)
=30^2-1^2
=900-1
=899
9.9x10.1
=(10-0.1)(10+0.1)
=10^2-0.1^2
=100-0.01
=99.99
98x102
=(100-2)(100+2)
=100^2-2^2
=10000-4
=9996
(11/9)*(7/9)
=(1+2/9)(1-2/9)
=1-(2/9)^2
=1-4/81
=77/81

31乘29=30^2-1^2=900-1=899
9.9乘10.1=10^2-.1^2=100-0.01=99.99
98乘102=100^2-2^2=10000-4=9996
九分之11乘九分之七=1^2-(2/9)^2=1-4/81=77/81