Question

tìm x biết

1+2+3+...+x=55

Answer 1

x = 10

Answer 2

1+2+3+...+x=55

\(\Rightarrow\frac{\left(n+1\right)n}{2}=55\)

=> (n + 1)n = 55 . 2

=> (n + 1)n = 110

=> (n + 1)n = 11.10

=> n = 10

Answer 3

1+2+3+...+x=55

\(\Rightarrow\frac{\left(x+1\right)x}{2}=55\)

=> (x + 1)x = 55.2

=> (x + 1)x = 110

Mà 110 = 11.10

=> x = 10

Answer 4

Theo mình thì x = 10