Tách 2 vế ta đươc : ( x + x + ...+ x ) + ( 1 + 2 + ... + 20 ) = 315 (=) 21x + 210 = 315 (=) 21x = 105 (=) x = 5
x + x + 1 + x + 2 + ...... + x + 20 = 315
21x + 210 = 315
21x = 315 - 210
21x = 105
x = 105 : 21
x = 5
=>(x+x+x+.............+x)+(1+2+.............+20)=315
=>21x+210=315
=>21x=315-210
=>21x=105
=>x=5
Ta có : x + ( x + 1 ) + ( x + 2 ) + ... + ( x + 20 ) = 315 .
=> [ x + ( x + 20 ) ] * { [ ( x + 20 ) - x ] : 1 + 1 } : 2 = 315 .
=> [ x + x + 20 ] * { [ x + 20 - x ] : 1 + 1 } = 315 * 2 .
=> [ x + x + 20 ] * { [ x + 20 - x ] : 1 + 1 } = 630 .
=> [ 2x + 20 ] * { 20 : 1 + 1 } = 630 .
=> [ 2x + 20 ] * 21 = 630 .
=> 2x + 20 = 630 : 21 .
=> 2x + 20 = 30 .
=> 2x = 30 - 20 .
=> 2x = 10 .
=> x = 10 : 2 .
=> x = 5 .
Vậy x = 5 .
(x+x+......+x)+(1+2+.....+20)=315
21 số hạng x
21x+210=315
21x=315-210=105
x=105/21=5
\(21\cdot x+\left[\left(20-1\right):1+1\right]\cdot\left(20+1\right):2=315\)
\(21\cdot x+\left[19+1\right]\cdot21:2=315\)
\(21\cdot x+20\cdot21:2=315\)
\(21\cdot x+10\cdot21=315\)
\(21\cdot x+210=315\)
\(21\cdot x=315-210\)
\(21\cdot x=105\)
\(x=105:21\)
\(x=5\)
