b) \(\dfrac{7}{3}.\dfrac{2}{7}-\dfrac{2}{7}.\dfrac{10}{3}-\dfrac{2}{7}=\dfrac{2}{3}-\dfrac{20}{21}-\dfrac{2}{7}=-\dfrac{4}{7}\)

b) \(B=x^2+5y^2+4xy+6x+6y-12\)

\(< =>\left(x+2y\right)^2+y^2+6\left(x+y\right)-12\)

\(< =>\left(x+2y\right)^2+6\left(x+2y\right)+y^2-6y-12\)

\(< =>\left(x+2y\right)^2+6\left(x+2y\right)+9+y^2=6y+9-30\)

\(< =>\left(x+2y+9\right)^2+\left(y-3\right)^2-30\)

Vay GTNN cua B=-30<=>x=-15,y=3

a) A=\(\dfrac{5}{4}x^2+xy+y^2-3x+5< =>\left(x+y\right)^2+\left(\dfrac{1}{2}x-3\right)^2-4\)

Vay GTNN cua A=-4 <=>x=6 ,y=-6

\(< =>2x-2\sqrt{x^2-\left(2x-1\right)}=2\)

<=>\(x-1=\sqrt{x^2-2x+1}\)

<=>x-1=|x-1|

\(< =>\left[{}\begin{matrix}x-1=1-x\\x-1=x-1\end{matrix}\right.< =>\left[{}\begin{matrix}x=1\\0x=0\end{matrix}\right.\)

=> x=1

e) 

\(\sqrt{x-2\sqrt{x-1}}-\sqrt{x-1}=1\)

<=>\(x-2\sqrt{x-1}=x +2\sqrt{x+1}\)

<=>\(-2\sqrt{x-1}=2\sqrt{x+1}\)(loai)

Vi \(\sqrt{A}\ge0\)

Cx nhanh ddaay

\(B=10<=> 2x^2+10x-3=10\)

\(< =>2x^2+10x-13=0\)

<=>\(\left[{}\begin{matrix}x=\dfrac{-5+\sqrt{51}}{2}\\x=\dfrac{-5-\sqrt{51}}{2}\end{matrix}\right.\)

51. B -> Nick's house

52. A -> has learnt

53. C -> on

54. D -> girls

55. D -> dancing