Phân số chỉ 24m vải là:

\(1-\dfrac{1}{5}-\dfrac{1}{3}\cdot\left(1-\dfrac{1}{5}\right)=\dfrac{8}{15}\) 

Độ dài của tấm vải ban đầu:

\(24:\dfrac{8}{15}=45\left(m\right)\)

\(\dfrac{15,6\times250-26\times78:0,5}{15,6\times3,2\times2,4\times2,6}\)

\(=\dfrac{15,6\times250-26\times78\times2}{15,6\times3,2\times2,4\times2,6}\)

\(=\dfrac{3900-4056}{311,5008}\)

\(=\dfrac{-156}{311,5008}\)

15,6×3,2×2,4×2,615,6×250−26×78:0,5​

=15,6×250−26×78×215,6×3,2×2,4×2,6=15,6×3,2×2,4×2,615,6×250−26×78×2​

=3900−4056311,5008=311,50083900−4056​

=−156311,5008=311,5008−156​

Phân số chỉ lượng vải chiều bán so với tổng độ dài vải ban đầu là:

\(\dfrac{3}{8}\times\left(1-\dfrac{3}{11}\right)=\dfrac{3}{11}\left(tấm.vải.ban.đầu\right)\)

Phân số chỉ lượng vải còn lại sau 2 lần bán so với tổng độ dài vải ban đầu là:

\(1-\left(\dfrac{3}{11}+\dfrac{3}{11}\right)=\dfrac{5}{11}\left(tấm.vải.ban.đầu\right)\)

Tấm vải ban đầu dài:

\(20:\dfrac{5}{11}=44\left(m\right)\)

Đáp số: 44 mét vải

giai lun a

\(\dfrac{1}{3}+\dfrac{2}{5}+\dfrac{2}{3}+\dfrac{2}{3}\)

\(=\left(\dfrac{1}{3}+\dfrac{2}{3}\right)+\left(\dfrac{2}{5}+\dfrac{2}{3}\right)\)

\(=\dfrac{3}{3}+\dfrac{16}{15}\)

\(=1+\dfrac{16}{15}\)

\(=\dfrac{15}{15}+\dfrac{16}{15}\)

\(=\dfrac{31}{15}\)

\(\dfrac{1}{3}+\dfrac{2}{5}+\dfrac{2}{3}+\dfrac{2}{3}=\left(\dfrac{1}{3}+\dfrac{2}{3}\right)+\dfrac{2}{5}+\dfrac{2}{3}=1+\dfrac{2}{5}+\dfrac{2}{3}=\dfrac{5}{3}+\dfrac{2}{5}=\dfrac{5\times5+2\times3}{15}=\dfrac{31}{15}\)

\(7\dfrac{19}{100}=7+\dfrac{19}{100}=7+0,19=7,19\\ Chọn.A\)

\(Bài.1:\\ a,0,125.\left(-3,7\right).2^3=0,125.\left(-3.7\right).8\\ =\left(0,125.8\right).\left(-3,7\right)=1.\left(-3,7\right)=-3,7\\ b,\sqrt{36}.\sqrt{\dfrac{25}{16}}+\dfrac{1}{4}=6.\dfrac{5}{4}+\dfrac{1}{4}=\dfrac{15}{2}+\dfrac{1}{4}=\dfrac{30}{4}+\dfrac{1}{4}=\dfrac{31}{4}\\ c,\sqrt{\dfrac{4}{81}}.\sqrt{\dfrac{25}{81}}-\dfrac{12}{5}\\ =\dfrac{2}{9}.\dfrac{5}{9}-\dfrac{12}{5}=\dfrac{10}{81}-\dfrac{12}{5}=\dfrac{10.5-12.81}{420}=-\dfrac{461}{210}\\ d,0,1.\sqrt{225}.\sqrt{\dfrac{1}{4}}=0,1.15.\dfrac{1}{2}=0,75\)

Bài 2:

\(a,\dfrac{1}{5}+x=\dfrac{2}{3}\\ x=\dfrac{2}{3}-\dfrac{1}{5}=\dfrac{10}{15}-\dfrac{3}{15}=\dfrac{7}{15}\\ ---\\ b,-\dfrac{5}{8}+x=\dfrac{4}{9}\\ x=\dfrac{4}{9}-\left(-\dfrac{5}{8}\right)=\dfrac{4}{9}+\dfrac{5}{8}=\dfrac{4.8+5.9}{72}=\dfrac{77}{72}\\ ---\\ c,\dfrac{13}{4}x+1\dfrac{1}{2}=-\dfrac{4}{5}\\ \dfrac{13}{4}x+\dfrac{3}{2}=-\dfrac{4}{5}\\ \dfrac{13}{4}x=-\dfrac{4}{5}-\dfrac{3}{2}=\dfrac{-4.2-3.5}{10}=-\dfrac{23}{10}\\ x=-\dfrac{23}{10}:\dfrac{13}{4}=-\dfrac{23}{10}.\dfrac{4}{13}=-\dfrac{46}{65}\\ ---\\ d,\dfrac{1}{4}+\dfrac{3}{4}x=\dfrac{3}{4}\\ \dfrac{3}{4}x=\dfrac{3}{4}-\dfrac{1}{4}=\dfrac{1}{2}\\ x=\dfrac{1}{2}:\dfrac{3}{4}=\dfrac{1}{2}.\dfrac{4}{3}=\dfrac{4}{6}=\dfrac{2}{3}\)

1) \(\left(x-3\right)^2-4=0\)

\(\Leftrightarrow\left(x-3-2\right)\left(x-3+2\right)=0\)

\(\Leftrightarrow\left(x-5\right)\left(x-1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=5\\x=1\end{matrix}\right.\)

2) \(x^2-2x=24\)

\(\Leftrightarrow x^2-2x-24=0\)

\(\Leftrightarrow x^2+4x-6x-24=0\)

\(\Leftrightarrow x\left(x+4\right)-6\left(x+4\right)=0\)

\(\Leftrightarrow\left(x+4\right)\left(x-6\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=6\\x=-4\end{matrix}\right.\)

Câu 3 số xấu rồi e

\(C=\left\{1;3;5;7;9;...;95;97;99\right\}\)

C={1;3;5;7;9;11;13;15;17;19;21;23;25;27;29;31;33;35;37;39;41;43;45;47;49; 51;53;55;57;59;61;63;65;67;69;71;73;75;77;79;81;83;85;87;89;91;93;95;97;99}