\(2x-\left(1\dfrac{1}{4}-3\dfrac{2}{3}-2\dfrac{1}{6}\right)=5\dfrac{3}{4}-9\dfrac{4}{5}-1\dfrac{1}{10}\\ 2x-\left(\dfrac{5}{4}-\dfrac{11}{3}-\dfrac{13}{6}\right)=\dfrac{23}{4}-\dfrac{49}{5}-\dfrac{11}{10}\\ 2x-\left(-\dfrac{55}{12}\right)=-\dfrac{103}{20}\\ 2x=-\dfrac{146}{15}\\ x=-\dfrac{73}{15}\)

a) \(3x^3+10x^2-8x=0\)

\(\Rightarrow x\left(3x^2+10x-8\right)=0\\ \Rightarrow x\left(x+4\right)\left(3x-2\right)=0\\ \Rightarrow\left[{}\begin{matrix}x=0\\x=-4\\x=\dfrac{2}{3}\end{matrix}\right.\)

b) \(\left(x^2+5\right)\left(x-2\right)+4=2x\)

\(\Rightarrow\left(x^2+5\right)\left(x-2\right)+4-2x=0\)

\(\Rightarrow\left(x^2+5\right)\left(x-2\right)-2\left(x-2\right)=0\\ \Rightarrow\left(x-2\right)\left(x^2+3\right)=0\\ \Rightarrow x-2=0\\ \Rightarrow x=2\)

(Do \(x^2+3>0,\forall x\))

Phân số chỉ số học sinh tổ 3 là: \(1-\dfrac{1}{3}-\dfrac{2}{5}=\dfrac{4}{15}\)