Bài 11: \(\dfrac{2a+13b}{3a-7b}=\dfrac{2c+13d}{3c-7d}\)

=>\(\left(2a+13b\right)\left(3c-7d\right)=\left(3a-7b\right)\left(2c+13d\right)\)

=>\(6ac-14ad+39bc-91bd=6ac+39ad-14bc-91bd\)

=>-14ad-39ad=-14bc-39bc

=>ad=bc

=>\(\dfrac{a}{b}=\dfrac{c}{d}\)

Bài 12:

\(\dfrac{a+2019}{a-2019}=\dfrac{b+2020}{b-2020}\)

=>\(\left(a+2019\right)\left(b-2020\right)=\left(a-2019\right)\left(b+2020\right)\)

=>\(ab-2020a+2019b-2019\cdot2020=ab+2020a-2019b-2019\cdot2020\)

=>-2020a-2020a=-2019b-2019b

=>2020a=2019b

=>\(\dfrac{a}{2019}=\dfrac{b}{2020}\)

Cho số người đội A;B;C lần lượt là a;b;c (a;b;c>0)

Theo bài ra ta có 

\(\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{4}\)và a + b + c = 130 

Theo tc dãy tỉ số bằng nhau 

\(\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{4}=\dfrac{a+b+c}{2+3+4}=\dfrac{130}{9}\)

- bn xem lại đề nhé

1: \(11=10+1=10^0+10^1\)

2: \(2485=2000+400+80+5=2\cdot10^3+4\cdot10^2+8\cdot10+5\cdot10^0\)

3: \(519=500+10+9=5\cdot10^2+10^1+9\cdot10^0\)

4: \(101=100+1=10^2+10^0\)

5: \(3278=3000+200+70+8=3\cdot10^3+2\cdot10^2+7\cdot10^1+8\cdot10^0\)

6: \(111=100+10+1=10^2+10^1+10^0\)

7: \(438=400+30+8=4\cdot10^2+3\cdot10^1+8\cdot10^0\)

8: \(112=100+10+2=10^2+10^1+2\cdot10^0\)

9: \(375=300+70+5=3\cdot10^2+7\cdot10^1+5\cdot10^0\)

10: \(1111=1000+100+10+1=10^3+10^2+10^1+10^0\)

a, Theo tc dãy tỉ số bằng nhau 

\(\dfrac{x}{3}=\dfrac{y}{5}=\dfrac{x-y}{3-5}=\dfrac{22}{-2}=-11\Rightarrow x=-33;y=-55\)

b, \(\dfrac{5}{2}=\dfrac{y}{x}\Rightarrow\dfrac{x}{2}=\dfrac{y}{5}\)Theo tc dãy tỉ số bằng nhau 

\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{x+y}{2+5}=-\dfrac{21}{7}=-3\Rightarrow x=-6;y=-15\)

c, \(7x=4y\Rightarrow\dfrac{x}{4}=\dfrac{y}{7}\)Theo tc dãy tỉ số bằng nhau 

\(\dfrac{x}{4}=\dfrac{y}{7}=\dfrac{x-y}{3-7}=-\dfrac{21}{-4}=7\Rightarrow x=28;y=49\)

Bài 5: Gọi số người của đội A,đội B,đội C lần lượt là a(người),b(người),c(người)

(Điều kiện: \(a,b,c\in Z^+\))

Tổng số người của ba đội là 130 người nên a+b+c=130

Số cây mỗi người của đội A;B;C trồng được lần lượt là 2;3;4 cây nên 2a=3b=4c

=>\(\dfrac{2a}{12}=\dfrac{3b}{12}=\dfrac{4c}{12}\)

=>\(\dfrac{a}{6}=\dfrac{b}{4}=\dfrac{c}{3}\)

Áp dụng tính chất của dãy tỉ số bằng nhau, ta được:

\(\dfrac{a}{6}=\dfrac{b}{4}=\dfrac{c}{3}=\dfrac{a+b+c}{6+4+3}=\dfrac{130}{13}=10\)

=>\(a=10\cdot6=60;b=4\cdot10=40;c=3\cdot10=30\)

Vậy: số người của đội A,đội B,đội C lần lượt là 60(người),40(người),30(người)

123+234=357

12345+9302747=9315092

456:12=38

Bài 8:

a: \(\dfrac{x}{5}=\dfrac{y}{6}\)

=>\(\dfrac{x}{20}=\dfrac{y}{24}\)

\(\dfrac{y}{8}=\dfrac{z}{7}\)

=>\(\dfrac{y}{24}=\dfrac{z}{21}\)

Do đó: \(\dfrac{x}{20}=\dfrac{y}{24}=\dfrac{z}{21}=k\)

=>x=20k;y=24k;z=21k

x+y-z=69

=>20k+24k-21k=69

=>23k=69

=>k=3

=>\(x=20\cdot3=60;y=24\cdot3=72;z=21\cdot3=63\)

b: Đặt \(\dfrac{x}{3}=\dfrac{y}{4}=\dfrac{z}{5}=k\)

=>x=3k;y=4k;z=5k

\(2x^2+2y^2-3z^2=-100\)

=>\(2\cdot\left(3k\right)^2+2\cdot\left(4k\right)^2-3\cdot\left(5k\right)^2=-100\)

=>\(k^2=4\)

=>\(\left[{}\begin{matrix}k=2\\k=-2\end{matrix}\right.\)

TH1: k=2

=>\(x=3\cdot2=6;y=4\cdot2=8;z=5\cdot2=10\)

TH2: k=-2

=>\(x=3\cdot\left(-2\right)=-6;y=4\cdot\left(-2\right)=-8;z=5\cdot\left(-2\right)=-10\)

Bài 10:

Đặt \(\dfrac{a}{b}=\dfrac{b}{c}=\dfrac{c}{d}=k\)

=>\(\left\{{}\begin{matrix}c=dk\\b=ck=dk\cdot k=dk^2\\a=bk=dk^2\cdot k=dk^3\end{matrix}\right.\)

a:

\(\left(\dfrac{a+b+c}{b+c+d}\right)^3=\left(\dfrac{dk^3+dk^2+dk}{dk^2+dk+d}\right)^3=\left(\dfrac{dk\left(k^2+k+1\right)}{d\left(k^2+k+1\right)}\right)^3=k^3\)

\(\dfrac{a}{d}=\dfrac{dk^3}{d}=k^3\)

Do đó: \(\dfrac{a}{d}=\left(\dfrac{a+b+c}{b+c+d}\right)^3\)

b: Sửa đề: Chứng minh \(\dfrac{a^3+c^3+b^3}{c^3+b^3+d^3}=\dfrac{a}{d}\)

 \(\dfrac{a^3+c^3+b^3}{c^3+b^3+d^3}=\dfrac{\left(dk^3\right)^3+\left(dk\right)^3+\left(dk^2\right)^3}{\left(dk\right)^3+\left(dk^2\right)^3+d^3}\)

\(=\dfrac{d^3k^9+d^3k^3+d^3k^6}{d^3k^3+d^3k^6+d^3}=\dfrac{d^3\cdot k^3\left(k^6+1+k^3\right)}{d^3\cdot\left(k^3+k^6+1\right)}=k^3\)

\(=\dfrac{dk^3}{d}=\dfrac{a}{d}\)

Bài 10:

Đặt \(\dfrac{a}{b}=\dfrac{b}{c}=\dfrac{c}{d}=k\)

=>\(\left\{{}\begin{matrix}c=dk\\b=ck=dk\cdot k=dk^2\\a=bk=dk^2\cdot k=dk^3\end{matrix}\right.\)

a:

\(\left(\dfrac{a+b+c}{b+c+d}\right)^3=\left(\dfrac{dk^3+dk^2+dk}{dk^2+dk+d}\right)^3=\left(\dfrac{dk\left(k^2+k+1\right)}{d\left(k^2+k+1\right)}\right)^3=k^3\)

\(\dfrac{a}{d}=\dfrac{dk^3}{d}=k^3\)

Do đó: \(\dfrac{a}{d}=\left(\dfrac{a+b+c}{b+c+d}\right)^3\)

b: Sửa đề: Chứng minh \(\dfrac{a^3+c^3+b^3}{c^3+b^3+d^3}=\dfrac{a}{d}\)

 \(\dfrac{a^3+c^3+b^3}{c^3+b^3+d^3}=\dfrac{\left(dk^3\right)^3+\left(dk\right)^3+\left(dk^2\right)^3}{\left(dk\right)^3+\left(dk^2\right)^3+d^3}\)

\(=\dfrac{d^3k^9+d^3k^3+d^3k^6}{d^3k^3+d^3k^6+d^3}=\dfrac{d^3\cdot k^3\left(k^6+1+k^3\right)}{d^3\cdot\left(k^3+k^6+1\right)}=k^3\)

\(=\dfrac{dk^3}{d}=\dfrac{a}{d}\)

Bài 14:

x+y+z=0

=>x+y=-z; x+z=-y; y+z=-x

\(A=\left(x+y\right)\left(y+z\right)\left(x+z\right)=\left(-z\right)\cdot\left(-x\right)\cdot\left(-y\right)\)

=-xyz

=-2

\(\dfrac{1}{3}.\)\(\dfrac{3}{5}.\)\(\dfrac{5}{12}+\dfrac{2}{6}.\)\(\dfrac{6}{10}.\)\(\dfrac{10}{24}+\dfrac{3}{9}.\)\(\dfrac{9}{15}.\)\(\dfrac{15}{36}\)

\(=\)\(\dfrac{1}{3}.\)\(\dfrac{3}{5}.\)\(\dfrac{5}{12}+\dfrac{1}{3}.\)\(\dfrac{3}{5}.\)\(\dfrac{5}{12}+\dfrac{1}{3}.\)\(\dfrac{3}{5}.\)\(\dfrac{5}{12}\)

\(=\)(\(\dfrac{1}{3}.\)\(\dfrac{3}{5}.\)\(\dfrac{5}{12}\))\(.3\)

\(=\dfrac{1}{12}.3\)

\(=\dfrac{3}{36}\)

\(\)