1)
项数=(末项-首项)/ 公差 + 1 = (100 - 5 )/ 5 + 1 = 20
求和=(首项+末项)*项数 / 2 = ( 5+100)* 20 / 2 = 1050
2)
=1+(3-2)+(5-4)+.+(1991-1990) = 1+ 1+1+.+ 1 = 996
3)
5 - (5*5 -5)/5 = 1
4)
(5+5)/5+5-5=2
5)
(5*5-5-5)/5=3
6)
125*27+54*125-125= 125*(27+54-1) = 125 * 80 = 10000
7)
(104*112*198)/(224*52*198)= (2*52*112*198) / (112*2*52*198) =1
8)
117684 / 12 = 9807