a, 1 + 2 + 3 + ... + x = 120
=> (x+1)x/2 = 120
=>x(x +1)=120.2=240
=>15.16 = 240
=>x=15
Vậy x=15
Phần b làm tương tự
c, x - ( 2/1.3 + 2/3.5 + 2/5.7 + ... + 2/53.55) = 3/5
=> x = 3/5 + ( 2/1.3 + 2/3.5 + 2/5.7 + ... + 2/53.55)
=> x = 3/5 + ( 1-1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ... + 1/53 - 1/55 )
=> x = 3/5 + ( 1- 1/55 )
=> x = 3/5 + 54/55
=> x = 87/55
Vậy x = 87/55
a) \(1+2+3+...+x=120\)
\(\Leftrightarrow\frac{x\left(x+1\right)}{2}=120\)
\(\Leftrightarrow x\left(x+1\right)=120\times2\)
\(\Leftrightarrow x\left(x+1\right)=240\)
\(\Leftrightarrow x\left(x+1\right)=15.16\)
\(\Leftrightarrow x=15\)
