Question

[Quỳnh Anh]

Answer 1

[subjects]

BÀI NÀY MÌNH THẤY HƠI GIỐNG BÀI CỦA LỚP 7 NHA BẠN !

 bài 4 : lập các cặp phân số bằng nhau từ các tích sau : 

a) 3 x 8 = 4 x 6

\(\dfrac{3}{4}=\dfrac{6}{8};\dfrac{3}{6}=\dfrac{4}{8};\dfrac{4}{3}=\dfrac{8}{6};\dfrac{6}{3}=\dfrac{8}{4}\)

b) (-4) x 5 = (-2) x 10

\(\dfrac{-4}{10}=\dfrac{-2}{5};\dfrac{-4}{-2}=\dfrac{10}{5};\dfrac{10}{-4}=\dfrac{5}{-2};\dfrac{-2}{-4}=\dfrac{5}{10}\)

bài 5 : tìm \(x\)

1) \(180\div x=-12\)

\(x=180\div\left(-12\right)\\ x=-15\)

2) \(-15-x=-\left(-7\right)\)

\(-15-x=7\\ x=-15-7=-\left(15+7\right)\\ x=-22\)

3) \(-129-\left(35-x\right)=55\)

\(35-x=\left(-129\right)-55=-\left(129+55\right)\\ 35-x=-184\\ x=35-\left(-184\right)=35+184\\ x=219\)

4) \(2\times\left(x-6\right)^2-1=49\)

\(2\times\left(x-6\right)^2=50\\ \left(x-6\right)^2=50\div2=25\)

BÂY GIỜ CHÚNG GTA XÉT 2 TRƯỜNG HỢP

*trường hợp 1

\(\left(x-6\right)^2=5^2\\ \Rightarrow x-6=5\\ x=5+6\\ x=11\)

*trường hợp 2

\(\left(x-6\right)^2=\left(-5\right)^2\\ \Rightarrow x-6=-5\\ x=\left(-5\right)+6\\ x=1\)

5) \(\dfrac{x}{-15}=\dfrac{2}{3}\)

\(\Rightarrow x\times3=2\times\left(-15\right)\\ \Rightarrow x\times3=-30\\ \Rightarrow x=-10\)

6) \(\dfrac{-2}{5}=\dfrac{-12}{x}\)

\(\Rightarrow\left(-2\right)\times x=\left(-12\right)\times5\\ \Rightarrow\left(-2\right)\times=-60\\ \Rightarrow x=\left(-60\right)\div\left(-2\right)\\ \Rightarrow x=30\)

7) \(\dfrac{6}{5}=\dfrac{-12}{x}\)

\(\Rightarrow6\times x=\left(-12\right)\times5\Rightarrow6\times x=-60\\ \Rightarrow x=\left(-60\right)\div6\\ \Rightarrow x=-10\)

8) \(\dfrac{-9}{x}=\dfrac{3}{4}\)

\(\Rightarrow\left(-9\right)\times4=3\times x\\ \Rightarrow-36=3\times x\\ \Rightarrow x=\left(-36\right)\div3\\ x=-12\)

9) \(\dfrac{6}{5}=\dfrac{-24}{12-x}\)

\(\Rightarrow6\times\left(12-x\right)=\left(-24\right)\times5\\ \Rightarrow6\times\left(12-x\right)=-120\\ \Rightarrow12-x=\left(-120\right)\div6\\ \Rightarrow12-x=-20\\ \Rightarrow x=12-\left(-20\right)=12+20\\ x=32\)

10) \(\dfrac{2}{x+2}=\dfrac{1}{5}\)

\(\Rightarrow2\times5=1\times\left(x+2\right)\\ \Rightarrow10=1\times\left(x+2\right)\\ \Rightarrow x+2=10\\ \Rightarrow x=10-2\\ x=8\)

11) \(\dfrac{4}{x-1}=\dfrac{3}{15}\)

\(\Rightarrow4\times15=3\times\left(x-1\right)\\ \Rightarrow60=3\times\left(x-1\right)\\ \Rightarrow x-1=20\\ \Rightarrow x=21\)