Tìm x biết:
1 + 2 + 3 + . . . + x = 210
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![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
-210=(-1)+(-2)+(-3)+...+(-x+1)+(-x)
=>-(1+2+3+...+x-1+x)=-210
=>1+2+3+...+x-1+x=210
=>x.(x+1):2=210
=>x(x+1)=210.2=420=20.21=20.(20+1)
=>x=20
vậy x=20
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
1+2+3+...+x=210
=> x.(x+1):2=210
=> x.(x+1)=210.2
=> x.(x+1)=420
=> x.(x+1)=20.21
=> x.(x+1)=20.(20+1)
=> x=20
1+2+3+...+x=420
=>x(x+1):2=210
=>x2+x-420=0
=>x2+21x-20x-420=0
=>x(x+21)-20(x+21)=0
=>(x-20)(x+21)=0
vì x>0=>x-20=0
=>x=20
vậy x=20
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:1+2+3+...+x=[x.(x+1)]:2=210
=>x.(x+1)=210.2
x.(x+1)=420=20.21
=>x=20
![](https://rs.olm.vn/images/avt/0.png?1311)
a)
x + (x + 1) + (x + 2) + ... + (x + 30) = 1240
x + x + 1 + x + 2 + ... + x + 30 = 1240
(x + x + ... + x) + (1 + 2 + ... + 30) = 1240
(x . [30 - 1 + 1 + 1]) + ([30 + 1] . [30 - 1 + 1] : 2) = 1240
31x + 465 = 1240
31x = 1240 - 465
31x = 775
\(a,x+\left(x+1\right)+\left(x+2\right)+...+\left(x+30\right)=1240\)
\(31x+1+2+3+...+30=1240\)
\(31x+465=1240\)
\(31x=775\)
\(x=25\)
![](https://rs.olm.vn/images/avt/0.png?1311)
1+2+3+.........+x=210
X x (X+1):2=210
X x (X+1) =420
20x21=420.Vậy x=20