Question

Tìm x biết 

1+2+3+4+.....+x=45

Answer 1

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

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

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

mà \(90=9.10\)

\(\Rightarrow x=9\)

Answer 2

1 + 2 + 3 + 4 + ... + x = 45

(1 + x) × x : 2 = 45

(1 + x) × x = 45 × 2

(1 + x) × x = 90

(1 + x) × x = 10 × 9

=> x = 9

Vậy x = 9