简便计算几道.
问题描述：
简便计算几道.
（80＋1/78）×79
（224＋1/222）×1/223
1/2＋1/6＋1/12＋……1/90
答
(80＋1/78）×79=80X79+1/78x79=80X(80-1)+1/78X(78+1)=80X80-80X1+1/78X78+1/78X1=640-80+1+1/78=561+1/78=561又1/78（224＋1/222）×1/223=（223+1+/222）X1/223=【223+（1+1/222）】X1/223=（223+223/222）X1/223...