Question

tim x 1+2+3+.......+.x= 45

Answer 1

theo công thức tính tổng dãy số cách đều ý bạn

(x+1).x:2=45

(x+1).x=90=10.9

=> x=9

Answer 2

1 + 2 + 3 + ... + x = 45

\(\Rightarrow\frac{x.\left(x+1\right)}{2}=45\)

\(\Rightarrow x.\left(x+1\right)=90\)

\(\Rightarrow x.\left(x+1\right)=9.10\)

\(\Rightarrow x=9\)

Answer 3

ta có 

\(1+2+3+4+...+x=45.\)

\(\frac{x\left(x+1\right)}{2}=45\)

\(x\left(x+1\right)=90\)

\(x\left(x+1\right)=9.10\)

=>\(x=9\)