Question

1.2+2.3+3.4+...+x(x+1)=29700

 

Answer 1

Đặt A = 1.2+2.3+3.4+...+x(x+1)

=> 3A = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ..... + x.(x + 1).(x + 2)

=> 3A = x.(x + 1).(x + 2) 

=> A=  x.(x + 1).(x + 2) : 3

Ta có:  x.(x + 1).(x + 2) : 3 = 29700

=>       x.(x + 1).(x + 2) = 89100

=>       x.(x + 1).(x + 2)  = 

Sai đề 

Answer 2

Đề sai nhé : mình sủa lại đề nha

1.2 + 2.3 + .... + x(x + 1) = 29760

\(\Leftrightarrow\frac{x\left(x+1\right)\left(x+2\right)}{3}=29760\)

\(\Leftrightarrow x\left(x+1\right)\left(x+2\right)=29760\)

\(\Leftrightarrow x\left(x+1\right)\left(x+2\right)=30.31.32\)

\(\Rightarrow x=30\)