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\(a,\frac{AB}{CD}=\frac{4}{12}=\frac{1}{3}\)

\(b,\) Đổi: \(12m=120dm\)

\(\frac{CD}{EF}=\frac{120}{20}=6\)

Các câu còn lại tương tự.

Gọi tử ban đầu là \(x\left(x\ne-3\right)\)

Mẫu ban đầu là \(x+3\)(đây là lí do tại sao \(x\ne-3\))

Tử lúc sau là \(x+2\)

Mẫu lúc sau là \(x+3+2=x+5\)

Theo đề bài, ta có: \(\frac{x+2}{x+5}=\frac{1}{2}\)

Đến đây em tự giải nhé. (cũng dễ rồi)

a) \(x\left(2x-9\right)=3x\left(x-5\right)\)

\(\Leftrightarrow x.\left(2x-9\right)-x.3\left(x-5\right)=0\)

\(\Leftrightarrow x.\left[\left(2x-9\right)-3\left(x-5\right)\right]=0\)

\(\Leftrightarrow x.\left(2x-9-3x+15\right)=0\)

\(\Leftrightarrow x.\left(6-x\right)=0\)

\(\Leftrightarrow S=\left\{0;6\right\}\)

b) \(0,5x\left(x-3\right)=\left(x-3\right)\left(1,5x-1\right)\)

\(\Leftrightarrow0,5x\left(x-3\right)-\left(x-3\right)\left(1,5x-1\right)=0\)

\(\Leftrightarrow\left(x-3\right).\left[0,5x-\left(1,5x-1\right)\right]=0\)

\(\Leftrightarrow\left(x-3\right)\left(0,5x-1,5x+1\right)=0\)

\(\Leftrightarrow\left(x-3\right)\left(1-x\right)=0\)

\(+x-3=0\Rightarrow x=3\)

\(+1-x=0\Rightarrow x=1\)

\(\Rightarrow S=\left\{1;3\right\}\)

c) \(3x-15=2x\left(x-5\right)\)

\(\Leftrightarrow\left(3x-15\right)-2x\left(x-5\right)=0\)

\(\Leftrightarrow3\left(x-5\right)-2x\left(x-5\right)=0\)

\(\Leftrightarrow\left(3-2x\right)\left(x-5\right)=0\)

\(\Rightarrow3-2x=\frac{3}{2}\Rightarrow x-5\Rightarrow x=5\)

\(\Rightarrow S=\left\{5;\frac{3}{2}\right\}\)

a)
\(x\left(2\times-9\right)=3\times\left(\times-5\right)\)

\(\text{⇔}x.\left(2\times-9\right)-x.3\left(x-5\right)=0\)

 \(\text{⇔}x.[\left(2\times-9\right)-3\left(x-5\right)]=0\)

 \(\text{⇔}x.\left(2x-9-3x+15\right)=0\)

 \(\text{⇔}x.\left(6-x\right)=0\)

\(\text{⇔}x=0\) hoặc \(6-x=0+6-x=0\)

\(\text{⇔}x=6\)

Vậy tập nghiệm của phương trình là \(S=\left\{0;6\right\}\) BIẾT MỖI CÂU A :))

\(A=\frac{5^2}{1.6}+\frac{5^2}{6.11}+...+\frac{5^2}{26.31}\)

   \(=5.\left(\frac{5}{1.6}+\frac{5}{6.11}+...+\frac{5}{26+31}\right)\)

   \(=5.\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+...+\frac{1}{26}-\frac{1}{31}\right)\)

    \(=5.\left(1-\frac{1}{31}\right)=5.\frac{30}{31}=\frac{150}{31}\)

Vậy...........

\(\frac{x-85}{15}+\frac{x-74}{13}+\frac{x-67}{11}+\frac{x-64}{9}=10\)

\(\Leftrightarrow\frac{x-85}{15}-1+\frac{x-74}{13}-2+\frac{x-67}{11}-3+\frac{x-64}{9}-4=0\)

\(\Leftrightarrow\frac{x-100}{15}+\frac{x-100}{13}+\frac{x-100}{11}+\frac{x-100}{9}=0\)

\(\Leftrightarrow x-100=0\)

\(\Leftrightarrow x=100\)

\(a,4x-6< 7x-12\)

\(\Leftrightarrow6< 3x\Leftrightarrow x>2\)

\(b,\frac{3x-7}{4}\ge2-\frac{x+5}{3}\)

\(\Leftrightarrow3\left(3x-7\right)\ge24-4\left(x+5\right)\)

\(\Leftrightarrow13x\ge25\Leftrightarrow x\ge\frac{25}{13}\)

\(c,\frac{3x-8}{-7}\ge1-\frac{x+2}{-3}\)

\(\Leftrightarrow-3\left(3x-8\right)\ge21+7\left(x+2\right)\)

\(\Leftrightarrow-16x\ge11\)

\(\Leftrightarrow x\le-\frac{11}{16}\)

\(d,-12-8x>3+2x-\left(5-7x\right)\)

\(\Leftrightarrow14>17x\Leftrightarrow x< \frac{14}{17}\)

\(e,-1+\frac{x-1}{-3}\le\frac{x+2}{-9}\)

\(\Leftrightarrow-9-3\left(x-1\right)\le-\left(x+2\right)\)

\(\Leftrightarrow-2x\le4\Leftrightarrow x\ge-2\)

`Answer:`