Question

Tìm x

1+4+7+10+....+x=330

Answer 1

Áp dụng công thức tính dãy số ta có :

\(\frac{\left[\left(x-1\right):3+1\right].\left(x+1\right)}{2}=330\)

\(\Rightarrow\left(\frac{x+1}{3}+1\right).\left(x+1\right)=660\)

\(\Rightarrow\left(\frac{x+4}{3}\right).\left(x+1\right)=660\)

\(\Rightarrow\frac{\left(x+4\right)\left(x+1\right)}{3}=660\)

\(\Rightarrow x.\left(x+1\right)+4.\left(x+1\right)=1980\)

\(\Rightarrow x^2+x+4x+4=1980\)

Answer 2

1+4+7+10+....+x=330

<=> ( x + 1 ) . x : 2 = 330 

hoặc x . ( x + 1 ) : 2 = 330

Answer 3

= 330

tíck mik nhé