Question

Tính giá trị của biểu thức

N=1/18+1/54+1/108+...+1/990

Answer 1

dễ ẹt N=1/18+...+1/990

N=1/3x6 + 1/6x9 + ...+ 1/30x33

N=1/3 - 1/6 + 1/6 - 1/9 +...+ 1/30 - 1/33

N=1/3 - 1/33 = 10/33

Answer 2

N=1/3.6+1/6.9+1/9.12+...+1/30.33

suy ra 3N=3/3.6+3/6.9+3/9.12+...+3/30.33

3N=1/3-1/6+1/6-1/9+1/9-1/12+...+1/30-1/33

3N=1/3-1/33=10/33

suy ra N= 10/33:3+10/99