Có số HS khối 8 là:200+25=225(HS)

P/s:Đề bài bị thiếu nên sợi chỉ đứt! Không tính dc số HS khối 6,7

\(\frac{1}{2.x}-\frac{1}{1.2}-\frac{1}{2.3}-......-\frac{1}{45.46}=-2\)2

\(\frac{1}{2.x}-\left(\frac{1}{1.2}+\frac{1}{2.3}+.......+\frac{1}{45.46}\right)=-2\)

Đặt \(A=\frac{1}{1.2}+\frac{1}{2.3}+.....+\frac{1}{45.46}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+.....+\frac{1}{45}-\frac{1}{46}\)

\(A=1-\frac{1}{46}=\frac{45}{46}\)

Ta có: \(\frac{1}{2.x}-\frac{45}{46}=-2\)

\(\frac{1}{2.x}=\frac{-47}{46}\)

\(\frac{-47}{-94.x}=\frac{-47}{46}\)

\(\Rightarrow x=\frac{-23}{47}\)

Biết a=b=c=d 

Thay vào M

Ta có: 

\(M=\frac{2a-a}{a+a}+\frac{2a-a}{a+a}+\frac{2a-a}{a+a}+\frac{2a-a}{a+a}\)

\(=4.\frac{2a-a}{a+a}=4.\frac{a}{2a}=4.\frac{1}{2}=2\)

\(\left[\left(3x+1\right)^3\right]^5=15^0\)

\(\Leftrightarrow\left(3x+1\right)^{15}=1\)

\(\Leftrightarrow\left(3x+1\right)^{15}=1^{15}\)

\(\Rightarrow3x+1=1\)

\(\Leftrightarrow3x=1-1\)

\(\Leftrightarrow3x=0\Rightarrow x=0\)

\(\left[(3\times+1)^3\right]^5=15^0\)

\(\Rightarrow\left[(3\times+1)^3\right]^5=1\)

\(\Rightarrow\left[(3\times+1)^3\right]^5=1^5\)

\(\Rightarrow(3\times+1)^3=1\)

\(\Rightarrow(3\times+1)^3=1^3\)

\(\Rightarrow3\times+1=1\)

\(\Rightarrow3\times=1-1\)

\(\Rightarrow3\times=0\)

\(\Rightarrow\times=0\)

2 . |​x-1| - 9 = -13 + 17 

2 . |​x-1| - 9 = 4 

     2 . |x-1| = 4 + 9

     2 . |x-1| = 13

          |x-1| = 13 : 2

          |x-1| = 13/2

=> x - 1 = 13 / 2 hoặc x - 1 = -13 / 2

+) x - 1 = 13 / 2

          x = 13 / 2 + 1

          x = 15 / 2

+) x - 1 = -13 / 2

         x = -13 / 2 + 1

         x = -11 / 2

Vậy x \(\in\) { 15 / 2 ; -11 / 2 }

\(2\cdot\left|x-1\right|-9=-13+17\)

\(\Leftrightarrow2\cdot\left|x-1\right|-9=5\)

\(\Leftrightarrow2\cdot\left|x-1\right|=5+9\)

\(\Leftrightarrow2\cdot\left|x-1\right|=14\)

\(\Leftrightarrow\left|x-1\right|=\frac{14}{2}\)

 \(\Leftrightarrow\left|x-1\right|=7\Leftrightarrow x-1=\pm7\)

TH1:\(x-1=17\)

\(\Leftrightarrow x=17+1=18\)

TH2:\(x-1=-17\)

\(\Leftrightarrow x=-17+1=-16\)

    Vậy x=18;-16

sai đề thì phải bạn ạ

nếu đề thế thì ko tính đc sau bao lâu 2 xe gặp nhau đâu

bạn xem lại đề ik 

a) \(326-2x=78\Rightarrow2x=326-78=248\)

\(\Rightarrow x=248:2=124\)

b) \(42.5-\left(3x+6\right)=3^4\Rightarrow210-\left(3x+6\right)=81\)

\(\Rightarrow3x+6=210-81=129\Rightarrow3x=129-6=123\)

\(\Rightarrow x=123:3=41\)

a, 326 -2x =78

\(\Rightarrow\)2x=326-78=248

\(\Rightarrow\)x=248/2=124

b,\(42.5-\left(3x+6\right)=3^4\)

\(\Rightarrow\)\(210-3x-6=81\)

\(\Rightarrow\)3x=123

\(\Rightarrow\)x=123/3=41

c, \(4^x:64=4^{250}\)

\(\Rightarrow\)\(4^x:4^3=4^{250}\)

\(\Rightarrow\)x=253

d, \(30x+1+2+3+...+30=495\)

\(\Rightarrow\)\(30x+465=495\)

\(\Rightarrow\)\(x=1\)