a)x.(5-2x)-2x.(1-x)=15
   x [ 5 - 2x -2.(1-x) ] = 15
   x ( 5 - 2x -2 + 2x ) =15
   x . 3 =15
   x = 5
b)(3x+2)²+(1+3x).(1-3x)=2
   9x²+12x+4+1-9x²=2
   12x + 5 = 2
    12x = -3
        x = -1/4

a)\(\Leftrightarrow\)\(5x-2x^2-2x+2x^2=15\)

\(\Leftrightarrow\)\(3x=15\)

\(\Leftrightarrow\)\(x=5\)

b)\(\Leftrightarrow\)\(9x^2+12x+4+1-9x^2-2=0\)

\(\Leftrightarrow\)\(12x+3=0\)

\(\Leftrightarrow\)\(x=-0,25\)

Bài 1:

a) \(x^2-6x+15=\left(x^2-6x+9\right)+6=\left(x-3\right)^2+6\ge6\)

Dấu "=" xảy ra \(\Leftrightarrow x=3\)

b) \(3x^2-15x+4=3\left(x^2-5x+\dfrac{25}{4}\right)-\dfrac{59}{4}=3\left(x-\dfrac{5}{2}\right)^2-\dfrac{59}{4}\ge-\dfrac{59}{4}\)

Dấu "=" xảy ra \(\Leftrightarrow x=\dfrac{5}{2}\)

Bài 2:

a) \(\Rightarrow\left(x-5\right)\left(x+5\right)+2\left(x+5\right)=0\)

\(\Rightarrow\left(x+5\right)\left(x-3\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=-5\\x=3\end{matrix}\right.\)

c) \(\Rightarrow x^2\left(x-2\right)+7\left(x-2\right)=0\)

\(\Rightarrow\left(x-2\right)\left(x^2+7\right)=0\)

\(\Rightarrow x=2\left(do.x^2+7\ge7>0\right)\)

\(a,25x^2-1=15\)\(< =>x^2=\dfrac{16}{25}< =>x=\pm\dfrac{4}{5}\)

\(b,\left(x-4\right)^2-\left(5x+2\right)^2=0\)\(< =>\left(-4x-6\right)\left(6x-2\right)=0\)

\(< =>\left[{}\begin{matrix}x=-\dfrac{3}{2}\\x=\dfrac{1}{3}\end{matrix}\right.\)

\(c,\left(x-1\right)\left(x-9\right)=0< =>\left[{}\begin{matrix}x=1\\x=9\end{matrix}\right.\)

a) Ta có: \(\left(2x-1\right)\left(x^2-x+1\right)=2x^3-3x^2+2\)

\(\Leftrightarrow2x^3-2x^2+2x-x^2+x-1-2x^3+3x^2-2=0\)

\(\Leftrightarrow3x=3\)

hay x=1

Vậy: S={1}

b) Ta có: \(\left(x+1\right)\left(x^2+2x+4\right)-x^3-3x^2+16=0\)

\(\Leftrightarrow x^3+2x^2+4x+x^2+2x+4-x^3-3x^2+16=0\)

\(\Leftrightarrow6x=-20\)

hay \(x=-\dfrac{10}{3}\)

c) Ta có: \(\left(x+1\right)\cdot\left(x+2\right)\left(x+5\right)-x^3-8x^2=27\)

\(\Leftrightarrow\left(x^2+3x+2\right)\left(x+5\right)-x^3-8x^2-27=0\)

\(\Leftrightarrow x^3+5x^2+3x^2+15x+2x+10-x^3-8x^2-27=0\)

\(\Leftrightarrow17x=17\)

hay x=1

Bài 2: 

a: 7/13:x=14/39

nên \(x=\dfrac{7}{13}:\dfrac{14}{39}=\dfrac{7}{13}\cdot\dfrac{39}{14}=\dfrac{3}{2}\)

b: x+3/7=2/5+3/10=4/10+3/10=7/10

=>x=7/10-3/7=49/70-30/70=19/70

a) \(\dfrac{2}{5}+\dfrac{11}{15}=\dfrac{6}{15}+\dfrac{11}{15}=\dfrac{17}{15}\)

b) \(\dfrac{7}{8}-\dfrac{7}{9}=\dfrac{63}{72}-\dfrac{56}{72}=\dfrac{7}{72}\)

c) \(\dfrac{11}{13}\cdot\dfrac{26}{31}=\dfrac{26}{13}\cdot\dfrac{11}{31}=\dfrac{22}{31}\)

d) \(\dfrac{1}{2}:\dfrac{1}{3}\cdot\dfrac{2}{5}=\dfrac{1}{2}\cdot3\cdot\dfrac{2}{5}=\dfrac{3}{5}\)

a ) 2/5 + 11/15 = 6/15 + 11/ 15 = 17/15.

b ) 7/8 -7/9 = 63/72 -56/72 = 7/72.

c ) 11/13 x 26/31 = 22/31

d ) 1/2 : 1/3 x 2/5= 1/2 x 3/1 x 2/5 = 3/5

đúng thì tick mình nhé!!! cảm ơn bạn

1) 2⁴.x - 3.5x = 5² - 2⁴

16x - 15x = 25 - 16

x = 9

2) 3².x + 2²x = 26.2² - 13

9x + 4x = 26.4 - 13

13x = 104 - 13

13x = 91

x = 91 : 13

x = 7

5) 6²x - 5²x = 11.2 - 11

36x - 25x = 22 - 11

11x = 11

x = 11 : 11

x = 1

3) 5²x - 2⁴x = 3⁴ - 16.3²

25x - 16x = 81 - 16.9

9x = -63

x = -63 : 9

x = -7

6) 7²x - 6²x = 13.2³ - 26

49x - 36x = 13.8 - 26

13x = 104 - 26

13x = 78

x = 78 : 13

x = 6

4) 7²x - 14x = 7².10 - 70

49x - 14x = 49.10 - 70

35x = 490 - 70

35x = 420

x = 420 : 35

x = 12

Cảm ơn bạn nha

a: \(\Leftrightarrow8x\left(x-3\right)\left(x+3\right)=0\)

hay \(x\in\left\{0;3;-3\right\}\)

b: \(\Leftrightarrow x^2-4x+4-x^2-2x+3=12\)

=>-6x=5

hay x=-5/6