Đại số lớp 7

a ) \(\frac{-2}{x}=\frac{x}{\frac{8}{25}}\)

\(\Leftrightarrow x^2=-2.-\frac{16}{25}\)

\(\Leftrightarrow x^2=\frac{16}{25}\) 

\(\Leftrightarrow\left[\begin{array}{nghiempt}x=\sqrt{\frac{16}{25}}\\x=-\sqrt{\frac{16}{25}}\end{array}\right.\Leftrightarrow\left[\begin{array}{nghiempt}x=\frac{4}{5}\\x=-\frac{4}{5}\end{array}\right.\)

Vậy ......................................

\(\left(a-2\right)^2=16\)

\(\Leftrightarrow\left(a-2\right)^2=\pm\sqrt{16}\)

\(\Leftrightarrow\left(a-2\right)=\pm4\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}a-2=4\\a-2=-4\end{array}\right.\Leftrightarrow\left[\begin{array}{nghiempt}a=6\\a=-2\end{array}\right.\)

Vậy \(a\in\left\{6;-2\right\}\)

\(\left(2x-3\right)^2=81\)

\(\Leftrightarrow\left(2x-3\right)^2=\pm\sqrt{81}\)

\(\Leftrightarrow\left(2x-3\right)=\pm9\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}2x-3=9\\2x-3=-9\end{array}\right.\Leftrightarrow\left[\begin{array}{nghiempt}2x=12\\2x=-6\end{array}\right.\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}x=6\\x=-3\end{array}\right.\)

Vậy \(x\in\left\{6;-3\right\}\)

(2x- 3)²= 81

(2x- 3)²=9²

2x-3= 9 2x-3= -9

2x = 9+3 2x = (-9)+3

2x = 12 2x = -6

x = 12:2 x = -6:2

x = 6 x = -3

VAY x = 6 va x = -3

x² = x⁵

<=> x² - x⁵ = 0

<=> x² - x².x³ = 0

<=> x²(1-x³) = 0

<=> \(\left[\begin{array}{nghiempt}x^2=0\\1-x^3=0\end{array}\right.\)

<=> \(\left[\begin{array}{nghiempt}x=0\\x=1\end{array}\right.\)

x=1,5

(9.0,08 + 0,7.0.08).(9.12,5 - 0,7.12,5) - 9,49

= 0,08(9+0,7).12,5(9-0,7) - 9,49

= 0,776.103,75 - 9,49

= 80,51 - 9,49

= 71,02

giúp mk đi các bn .

help me

(9.0,08+0,7.0,08).(9.12,5-0,7.12,5)-9,49

\(\frac{2x-1}{2x+3}=\frac{x-5}{x+1}\)

\(\Rightarrow\frac{2x-1}{2x+3}-\frac{x-5}{x+1}=0\)

\(\Rightarrow\frac{\left(2x-1\right)\left(x+1\right)}{\left(2x+3\right)\left(x+1\right)}-\frac{\left(2x+3\right)\left(x-5\right)}{\left(2x+3\right)\left(x+1\right)}=0\)

\(\Rightarrow\frac{2x^2+x-1-2x^2+7x+15}{\left(2x+3\right)\left(x+1\right)}=0\)

\(\Rightarrow\frac{8x+14}{\left(2x+3\right)\left(x+1\right)}=0\)

\(\Rightarrow8x+14=0\)

\(\Rightarrow8x=-14\)

\(\Rightarrow x=-\frac{7}{4}\)

cách đơn giản hơn thì nhân chéo 

\(\frac{x-2}{x-1}=\frac{x+4}{x+7}\)

\(\Rightarrow\left(x-2\right)\left(x+7\right)=\left(x-1\right)\left(x+4\right)\)

\(\Rightarrow x^2+5x-14=x^2+3x-4\)

\(\Rightarrow2x=10\)

\(\Rightarrow x=5\)

\(\frac{x-1}{x+2}=\frac{x-2}{x+3}\)

\(\Rightarrow\left(x-1\right)\left(x+3\right)=\left(x+2\right)\left(x-2\right)\)

\(\Rightarrow x^2+2x-3=x^2-4\)

\(\Rightarrow2x=-1\)

\(\Rightarrow x=-\frac{1}{2}\)

\(\frac{x-1}{x+2}=\frac{x-2}{x+3}\)

\(\Leftrightarrow\left(x-1\right).\left(x+3\right)=\left(x+2\right).\left(x-2\right)\)

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

\(\Leftrightarrow2x=-1\)

\(\Leftrightarrow x=-\frac{1}{2}\)

Chúc bạn học tốt 

\(\left(x+4\right):20=5:\left(x+4\right)\)

\(\Leftrightarrow\left(x+4\right).\left(x+4\right)=5.20\)

\(\Leftrightarrow\left(x+4\right)^2=100\)

\(\Leftrightarrow\left(x+4\right)^2=\pm\sqrt{100}\)

\(\Leftrightarrow\left(x+4\right)=\pm10\)

\(\Leftrightarrow\left[\begin{array}{nghiempt}x+4=10\\x+4=-10\end{array}\right.\Leftrightarrow\left[\begin{array}{nghiempt}x=6\\x=-14\end{array}\right.\)

Vậy\(x\in\left\{6;-14\right\}\)

Chúc bạn học tốt !!!!!