ta có :
3S= 1.2.3+2.3.3+3.4.3+4.5.3+....+99.100.3
=>3S=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+99.100.(101-98)
=>3S=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+99.100.101-98.99.100
=>3S=99.100.101
=>S=99.100.101:3=333300
Đúng 0
Bình luận (0)