Question

tìm x biết  1+2+3+...+x=55

Answer 1

1 + 2 + 3 +......+ x = 55 
<=> (1 + 9) + (2 + 8) + (3 + 7) + (4 + 6) + 5 +....+ x = 55 
<=> x = 55 - [(1 + 9) + (2 + 8) + (3 + 7) + (4 + 6) + 5 +....] 
<=> x = 55 - (45 + ...) 
<=> x = 10 - (....) 

Nếu x > 0 => x = 10 
___ x < 0 => x = {-10; -20;....}

Tk nhé chúc bn hok giỏi

Answer 2

1 + 2 + 3 + .... + x = 55 

Ta có : 

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

\(\Leftrightarrow\left(1+x\right).x=55.2\)

\(\Leftrightarrow\left(1+x\right).x=110\) 

\(\Leftrightarrow\left(1+x\right).x=11.10\)

\(\Leftrightarrow x=10\)