Question

a, tinh tong D=1.2+2.3+3.4+...+99.100

b, su dung ket qua cau a tinh nhanh:

E=1²+2²+3²+...+98²+99²

^{c, tinh nhanh: F=1.100+2.99+3.98+...+98.3+99.2+100.1}

     ^{minh can lam giup minh voiiii....}

 

Answer 1

D = 1.2 + 2.3 + 3.4 + ... + 99.100

3D = 1.2.3 + 2.3.3 + 3.4.3 + ... + 99.100.3

3D = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 99.100.(101 - 98)

3D = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 99.100.101 - 98.99.100

3D = 99.100.101

D = 99.100.101 : 3 = ...