2 tá \(=24\)

Muốn mua 8 cái bút chì cần trả \(30000:24\times8=10000\left(VNĐ\right)\)

Số tiền phải trả khi mua 8 cái bút là:

\(30000:24\cdot8=10000\left(đồng\right)\)

10:Trong vở ghi của bn!

11:

S S' Mắt

Ta có khoảng cách từ vật đến gương phẳng bằng khoảng cách từ ảnh ảo đến gương 

\(\Rightarrow S'\) cách gương : 2cm

Bài 8: 

a) \(\left(-3,5\right):\left(-2\dfrac{3}{5}\right)=\dfrac{7}{2}:\dfrac{13}{2}=\dfrac{7}{2}\cdot\dfrac{2}{13}=\dfrac{7\cdot2}{2\cdot13}=\dfrac{7}{13}\)

b) \(\left(-\dfrac{11}{15}\right):1\dfrac{1}{10}=\left(-\dfrac{11}{15}\right):\dfrac{11}{10}=\left(-\dfrac{11}{15}\right)\cdot\dfrac{10}{11}=\dfrac{-11\cdot10}{15\cdot11}=-\dfrac{10}{15}=-\dfrac{2}{3}\)

c) \(2\dfrac{2}{3}:\left(-3\dfrac{3}{4}\right)=\dfrac{8}{3}:-\dfrac{15}{4}=\dfrac{8}{3}\cdot-\dfrac{4}{15}=\dfrac{8\cdot4}{3\cdot15}=-\dfrac{32}{45}\)

Bài 7:

a) \(\left(-\dfrac{3}{25}\right):6=\left(-\dfrac{3}{25}\right)\cdot\dfrac{1}{6}=\dfrac{-3\cdot1}{25\cdot6}=-\dfrac{1}{50}\)

b) \(-\dfrac{5}{23}:-2=\dfrac{5}{23}\cdot\dfrac{1}{2}=\dfrac{5\cdot1}{23\cdot2}=\dfrac{5}{26}\)

c) \(\dfrac{-7}{11}:-3,5=\dfrac{7}{11}:\dfrac{7}{2}=\dfrac{7}{11}\cdot\dfrac{2}{7}=\dfrac{7\cdot2}{11\cdot7}=\dfrac{2}{11}\)

a.=-2x³y² có bậc là:5

b.=15x⁴y⁴z có bậc là:9

c.=-2/3x³y⁴ có bậc là 7

d.=-2x⁶y⁶ có bậc là:12

đợi mik đi ăn cơm xong làm cho h mik off rùi :((

BÀi 2:

\(\left|x-\frac{2019}{2020}\right|\ge0\forall x\)

=>\(-\frac{2020}{2019}\left|x-\frac{2019}{2020}\right|\le0\forall x\)

=>\(-\frac{2020}{2019}\left|x-\frac{2019}{2020}\right|+\frac{2019}{2020}\le\frac{2019}{2020}\forall x\)

Dấu '=' xảy ra khi \(x-\frac{2019}{2020}=0\)

=>\(x=\frac{2019}{2020}\)

Bài 1:

a: \(A=\frac{155-\frac{10}{7}-\frac{5}{11}+\frac{5}{23}}{402-\frac{26}{7}-\frac{13}{11}+\frac{13}{23}}+\frac{\frac35+\frac{3}{13}-0,9}{\frac{7}{91}+0,2-\frac{3}{10}}\)

\(=\frac{5\left(31-\frac27-\frac{1}{11}+\frac{1}{23}\right)}{13\left(31-\frac27-\frac{1}{11}+\frac{1}{23}\right)}+\frac{3\left(\frac15+\frac{1}{13}-\frac{3}{10}\right)}{\frac15+\frac{1}{13}-\frac{3}{10}}=\frac{5}{13}+3=\frac{44}{13}\)

b: \(B=-\frac54+\frac35-1\frac{3}{14}:\left|-\frac{34}{21}\right|+\frac{-5}{17}:\left|-\frac{1}{34}\right|+\frac53\cdot\left(\frac12-\frac25\right)\)

\(=-\frac54+\frac35-\frac{17}{14}\cdot\frac{21}{34}+\frac{-5}{17}\cdot34+\frac53\cdot\frac{1}{10}\)

\(=-\frac54+\frac35-\frac{3}{2\cdot2}+\left(-10\right)+\frac{5}{30}=-\frac54+\frac35-\frac34+\left(-10\right)+\frac16\)

\(=-\frac{75}{60}+\frac{36}{60}-\frac{45}{60}+\frac{\left(-600\right)}{60}+\frac{10}{60}=\frac{-674}{60}=-\frac{337}{30}\)

c: \(C=1-\frac{1}{5\cdot10}-\frac{1}{10\cdot15}-\cdots-\frac{1}{95\cdot100}\)

\(=1-\frac15\left(\frac{5}{5\cdot10}+\frac{5}{10\cdot15}+\cdots+\frac{5}{95\cdot100}\right)\)

\(=1-\frac15\left(\frac15-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+\cdots+\frac{1}{95}-\frac{1}{100}\right)\)

\(=1-\frac15\left(\frac15-\frac{1}{100}\right)=1-\frac15\cdot\frac{19}{100}=1-\frac{19}{500}=\frac{481}{500}\)

d: \(D=\frac17+\frac{1}{91}+\frac{1}{247}+\frac{1}{475}+\frac{1}{775}+\frac{1}{1147}\)

\(=\frac{1}{1\cdot7}+\frac{1}{7\cdot13}+\cdots+\frac{1}{31\cdot37}\)

\(=\frac16\left(\frac{6}{1\cdot7}+\frac{6}{7\cdot13}+\cdots+\frac{6}{31\cdot37}\right)=\frac16\left(1-\frac17+\frac17-\frac{1}{13}+\cdots+\frac{1}{31}-\frac{1}{37}\right)\)

\(=\frac16\left(1-\frac{1}{37}\right)=\frac16\cdot\frac{36}{37}=\frac{6}{37}\)

Câu 4: 

Số đo các góc còn lại là \(47^0;133^0;133^0\)

1: Ta có: \(x^2-16=0\)

\(\Leftrightarrow\left(x-4\right)\left(x+4\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=4\\x=-4\end{matrix}\right.\)

2: Ta có: \(1-36x^2=0\)

\(\Leftrightarrow\left(6x-1\right)\left(6x+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{1}{6}\\x=-\dfrac{1}{6}\end{matrix}\right.\)

\(7,=\left(0,5a+5b\right)\left(0,25a^2-2,5ab+25b^2\right)\\ 8,=\left(a+b-c\right)\left(a^2+2ab+b^2+ac+bc+c^2\right)\\ 9,=\left(5+a-b\right)\left(25-5a+5b+a^2-2ab+b^2\right)\)

\(61,\\ 1,\Leftrightarrow\left(x-4\right)\left(x+4\right)=0\Leftrightarrow\left[{}\begin{matrix}x=4\\x=-4\end{matrix}\right.\\ 2,\Leftrightarrow\left(1-6x\right)\left(1+6x\right)=0\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{1}{6}\\x=-\dfrac{1}{6}\end{matrix}\right.\\ 3,\Leftrightarrow\left(6-x\right)\left(6+x\right)=0\Leftrightarrow\left[{}\begin{matrix}x=6\\x=-6\end{matrix}\right.\)

Bài 3: 

Ta có: a//b

nên \(x+y=180\)

mà \(2x-3y=0\)

nên \(\left\{{}\begin{matrix}x+y=180\\2x-3y=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x+2y=180\\2x-3y=0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}5y=180\\x+y=180\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=36\\x=144\end{matrix}\right.\)