Hiệu số phần bằng nhau là: 3 - 1 = 2 (lần)

Số học sinh giỏi là: 140 : 2 x 1 = 70 (học sinh)

Số học sinh khá là: 140 + 70 = 210 (học sinh)

\(2\times3^{x+1}-3^x=135\\ 2\times3\times3^x-3^x=135\\ 6\times3^x-3^x=135\\ 5\times3^x=135\\ 3^x=27\\ 3^x=3^3\\ x=3\)

\(15:x=5\\ x=15:5\\ x=3\)

\(\dfrac{x}{4}=3\\ x=3\times4\\ x=12\)

\(\dfrac{21}{x}=7\\ x=\dfrac{21}{7}\\ x=3\)

\(\sin\left(x+\dfrac{\pi}{7}\right)+\sin\left(\dfrac{3\pi}{2}-\dfrac{\pi}{5}\right)=0\\ \Rightarrow\sin\left(x+\dfrac{\pi}{7}\right)+\sin\left(\dfrac{13\pi}{10}\right)=0\\ \Rightarrow\sin\left(x+\dfrac{\pi}{7}\right)=-\sin\left(\dfrac{13\pi}{10}\right)=sin\left(-\dfrac{13\pi}{10}\right)\\ \Rightarrow\left[{}\begin{matrix}x+\dfrac{\pi}{7}=-\dfrac{13\pi}{10}+k2\pi\\x+\dfrac{\pi}{7}=\pi+\dfrac{13\pi}{10}+k2\pi\end{matrix}\right.\\ \Rightarrow\left[{}\begin{matrix}x=-\dfrac{101\pi}{70}+k2\pi\\x=\dfrac{151\pi}{70}+k2\pi\end{matrix}\right.\)

\(a:\dfrac{1}{6}=\dfrac{1}{3}-\dfrac{1}{5}\\ a:\dfrac{1}{6}=\dfrac{2}{15}\\ a=\dfrac{1}{45}\)

a) \(\dfrac{5}{6}+\dfrac{5}{8}+\dfrac{3}{4}=\dfrac{20}{24}+\dfrac{15}{24}+\dfrac{18}{24}=\dfrac{53}{24}\)

b) \(\dfrac{12}{15}+3+\dfrac{3}{5}=\dfrac{4}{5}+\dfrac{15}{5}+\dfrac{3}{5}=\dfrac{22}{5}\)

c) \(\dfrac{17}{9}-\dfrac{1}{6}-\dfrac{2}{3}=\dfrac{34}{18}-\dfrac{3}{18}-\dfrac{12}{18}=\dfrac{19}{18}\)

d) \(\dfrac{9}{5}+\dfrac{3}{4}-2=\dfrac{36}{20}+\dfrac{15}{20}-\dfrac{40}{20}=\dfrac{31}{20}\)