\(a,\left\{{}\begin{matrix}x+2y=5\\3x+4y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2y\\3\left(5-2y\right)+4y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2y\\15-6y+4y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2y\\15-2y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2.5\\y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=-5\\y=5\end{matrix}\right.\)

\(b,\left\{{}\begin{matrix}3x+y=3\\2x-y=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3x\\2y-\left(3-3x\right)=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3x\\2y-3+3x=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3x\\5x-3=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3.2\\x=2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=-3\\x=2\end{matrix}\right.\)

1. A--> when

2.D-->once

3.C-->but

4.A-->didn't watch

5.D-->mine

6.A-->who

Nửa chu vi tấm poster là:
\(\dfrac{32}{10}:2=\dfrac{8}{5}\left(m\right)\)

Chiều dài tấm poster là:

\(\left(\dfrac{8}{5}+\dfrac{2}{5}\right):2=1\left(m\right)\)

Chiều rộng tấm poster là:

\(\dfrac{8}{5}-1=\dfrac{3}{5}\left(m\right)\)

Diện tích tấm poster là:

\(1\times\dfrac{3}{5}=\dfrac{3}{5}\left(m^2\right)\)

Đáp số: \(\dfrac{3}{5}m^2\)

Từ 1 đến 9 dùng 9 chữ số
Từ 10 đến 99 dùng [(99-10):1+1]x2=180(chữ số)

Từ 100 đến 257 dùng [(257-100):1+1]x3=474(chữ số)

Cần dùng số chữ số là:

9+180+474=663(chữ số)

1, thay x=2 vào A ta có:
\(A=2^2-5.2+8=4-10+8=2\)

2, thay x=1,y=-1 vào B ta có:

\(B=1^3.\left(-1\right)+3.1.\left(-1\right)-\left(-1\right)^3\\ =-1-3+1\\ =-3\)

\(2,\left(x-\dfrac{1}{2}\right)\left(x^2+5\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x-\dfrac{1}{2}=0\\x^2+5=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=\dfrac{1}{2}\\x^2=-5\left(ktm\right)\end{matrix}\right.\)

\(a,y+\dfrac{2}{4}=2\\ y=2-\dfrac{1}{2}\\ y=\dfrac{4}{2}-\dfrac{1}{2}\\ y=\dfrac{3}{2}\\ b,\dfrac{3}{7}-y=\dfrac{2}{7}\times\dfrac{1}{2}\\ \dfrac{3}{7}-y=\dfrac{1}{7}\\ y=\dfrac{3}{7}-\dfrac{1}{7}\\ y=\dfrac{2}{7}\)

\(\dfrac{5}{2}-\dfrac{1}{3}:\dfrac{1}{4}\\ =\dfrac{5}{2}-\dfrac{1}{3}\times4\\ =\dfrac{5}{2}-\dfrac{4}{3}\\ =\dfrac{15}{6}-\dfrac{8}{6}\\ =\dfrac{7}{6}\)

A

\(\dfrac{5}{2}\times\dfrac{1}{3}+\dfrac{1}{4}\\ =\dfrac{5}{6}+\dfrac{1}{4}\\ =\dfrac{10}{12}+\dfrac{3}{12}\\ =\dfrac{13}{12}\)

\(\dfrac{5}{11}+\dfrac{13}{21}+\dfrac{6}{11}+\dfrac{8}{21}\\ =\left(\dfrac{5}{11}+\dfrac{6}{11}\right)+\left(\dfrac{13}{21}+\dfrac{8}{21}\right)\\ =\dfrac{11}{11}+\dfrac{21}{21}\\ =1+1\\ =2\)