运用分配律,结合律
例如;(2/3+1/6)*6=(2/3)*6+(1/6)*6=5
( 9/10)*(5/4)*22=( 9/10)*(5/4)*(20+2)
=( 9/10)*(5/4)*20+( 9/10)*(5/4)*2
=99/4