giải:
Đặt A=1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100
4A=(1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100)4
4A=1.2.3(4-0)+2.3.4(5-1)+3.4.5(6-2)+4.5.6(7-3)+...+98.99.100(101-97)
4A=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+4.5.6.7-3.4.5.6+...+98.99.100.101-97.98.99.100
4A=1.2.3.4-1.2.3.4+2.3.4.5-2.3.4.5+3.4.5.6-3.4.5.6+...+97.98.99.100-97.98.99.100+98.99.100.101
4A=98.99.100.101
=>A=98.99.100.101/4
Tính:
1.2.3+2.3.4+3.4.5+4.5.6+...+97.98.99+98.99.100
Tính tổng:
A=1.2.3+2.3.4+3.4.5+...+97.98.99
Tính nhanh 1.2.3+2.3.4+3.4.5+4.5.6+...+98.99.100.
Tính nhanh:
\(A=\frac{1}{1.2.3}-\frac{1}{2.3.4}-\frac{1}{3.4.5}-...-\frac{1}{97.98.99}\)
Tính nhanh:
a, A= 1.3+5.7+9.11+...+97.99
b, B= 1.2.3+2.3.4+3.4.5+...+98.99.100
Tính nhanh: D= 1/1.2.3+1/2.3.4 +1/3.4.5+ ..... + 1/98.99.100
tính 1.2.3+2.3.4+3.4.5+....+98.99.100
Tính : a, 1.2.3 + 2.3.4 + 3.4.5 + ... + (n - 1).n.(n+1)
b, 1.2.3 + 3.4.5 + 5.6.7 + 98.99.100
Tính A = 1/1.2.3 + 1/2.3.4 + 1/3.4.5 + ............. + 1/ 98.99.100
