giả sử: A = n(n+1) , có 2 trường hợp:

nếu n chẵn thì n chia hết cho 2 do đó A chia hết chia 2

nếu n lẻ thì n+1 chẵn do đó n+1 chia hết cho 2 nên A chia hết cho 2

`70-5(x-3)=45`

`=>5(x-3)=70-45=25`

`=>x-3=25:5=5`

`=>x=5+3=8`

`70-5.(x-3)=45`

`5.(x-3)=70-45`

`5.(x-3)=25`

`x-3=25:5`

`x-3=5`

`x=5+3`

`x=8`

`8.(3-x)=0`

`3-x=0:8`

`3-x=0`

`x=3-0`

`x=3`

\(8.\left(3-x\right)=0\)

\(3-x=0:8\)

\(3-x=0\)

\(x=3-0\)

\(x=3\)

#Hphong

\(\dfrac{1230}{3}.(x-20)=10\)

\(=>x-20=10:\dfrac{1230}{3}\)

\(=>x-20=\dfrac{1}{41}\)

\(=>x=\dfrac{1}{41}+20=\dfrac{821}{41}\)

\(\dfrac{1230}{3.\left(x-20\right)}=10\)

`3.(x-20)=1230:10`

`3.(x-20)=123`

`x-20=123:2`

`x-20=41`

`x=41+20`

`x=61`

Vậy `x=61`

`4 2/3 - (3/5 : x)=20%`

`4 2/3 - (3/5 : x)= 20/100`

`4 2/3 - (3/5 : x)=1/5`

`14/3 - (3/5 : x)=1/5`

`3/5:x=14/3-1/5`

`3/5:x=67/15`

`x=3/5 : 67/15`

`x=3/5 xx 15/67`

`x=9/67`

Vậy.... 

`#LeMichael`

\(4 \dfrac{2}{3}-(\dfrac{3}{5}:x)=20\)%

\(=>\dfrac{14}{3}-(\dfrac{3}{5}:x)=\dfrac{1}{5}\)

\(=>\dfrac{3}{5}:x=\dfrac{14}{3}-\dfrac{1}{5}\)

\(=>\dfrac{3}{5}:x=\dfrac{67}{15}\)

\(=>x=\dfrac{3}{5}:\dfrac{67}{15}\)

\(=>x=\dfrac{9}{67}\)

3,8 hay 3 và 8?

`2.(x+4)+5=65`

`=>2.(x+4)=65-5=60`

`=>x+4=60:2=30`

`=>x=30-4`

`=>x=26`

Tìm x

2 . (x + 4) + 5 = 65

2 . (x + 4) = 65 - 5

2 . (x + 4) = 60

(x + 4) = 60 : 2

(x + 4) = 30

x = 30 - 4

x = 26.