1+2+3+...+x=78

=>(x+1)x:2=78

=>x²+x=156

=>x²+13x-12x-156=0

=>x(13+x)-12(x+13)=0

=>(x-12)(x+13)=0

=>x-12=0 hoặc x+13=0

=>x=12 hoặc x=-13

vì x>0 =>x=12

vậy x=12

Ta có :

\(x\div\dfrac{41}{3}=-2,5\)

\(\Leftrightarrow x=\left(-2,5\right)\times\dfrac{41}{3}\)

\(\Leftrightarrow x=-\dfrac{205}{6}\)

Vậy \(x=-\dfrac{205}{6}\)

205/6 nha bn

\(x:4\dfrac{1}{3}=-2,5\)

\(\Rightarrow x:\dfrac{13}{3}=\dfrac{-25}{10}\)

\(\Rightarrow x=\dfrac{-25}{10}.\dfrac{13}{3}\)

\(\Rightarrow x=\dfrac{-65}{6}\)

\(1+2+3+.......+x=78\)

\(=>x=72\)

mik ko chắc nữa nếu sai thì xin lỗi nha 

Vì có mấy cái dấu chấm khó quá

Từ 1->x có: x-1+1=x (số hạng)

Ta có: \(1+2+3+...+x=78=>\frac{\left(x+1\right).x}{2}=78=>\left(x+1\right).x=156=12.13\)

=>x=12

Vậy x=12

1+2+3+...........+x=78

(x+1).x:2=78

(x+1).x=156

Ta có:156=12.13

=>x=12

Vậy x=12

x :41/2 = -2.5

        x= -2.5 * 41/2

        x= -51.25

X CHIA 4\13=-2\5

=>X=-2,5*4\13

=X=51,25

34,65 + 21,78 + 65,35 + 78,22

= (34,65 + 65,35) + (21,78 + 78,,22)

= 100 + 100

= 200

4,5 x 5,5 + 4,7 x 4,5

= 4,5 x (5,5 + 4,5)

= 4,5 x 10

= 45

7,5 x 2,5 x 4

= 7,5 x 10

= 75

\(34,65+21,78+65,35+78,22=\) 

\(=\left(34,65+65,35\right)+\left(21,78+78,22\right)\)

\(=100+100\)

\(=200\)

\(4,5\times5,5+4,7\times4,5=\)

\(=4,5\times\left(5,5+4,7\right)\)

\(=4,5\times10,2\)

\(=45,9\)

 \(7,5\times2,5\times4=\)

\(=7,5\times\left(2,5\times4\right)\)

\(=7,5\times10\)

\(=75\)

(x-3,64)*2,5=15,85

=>x-3,64=15,85/2,5=6,34

=>x=6,34+3,64=10

52*(y/78)=3380

=>y/78=3380/52

=>y/78=65

=>y=65*78

=>y=5070

X:2+x+x:3+x:4=25

3:

a: =100+100=200

b: =4,5(5,5+4,7)

=4,5*10,2

=45,9

c: =7,5*10=75

a, |-2,5 + x| = 1,3

=> -2,5 + x = 1,3 hoặc -2,5 + x = -1,3

=> x = 3,8 hoặc x = 1,2

vậy_

b,