请采纳
7.9x2.1+2.1x2.1的简便计算=（7.9+2.1）x2.1=10x2.1=21
1/1x2+1/2x3+1/3x4省略+1/9x10 =1-1/2+1/2-1/3+1/3-1/4+......+1/9-1/10 =1-1/10 =9/10
