a. \(x\ne5\) là ĐKXĐ của biểu thức P

b. P =\(\dfrac{\left(x-5\right)^2}{x-5}\)=\(x-5\)

c. P = -1 <=> x-5 =-1 <=> x=4

\(\dfrac{x}{x-5}+\dfrac{4x}{x+5}+\dfrac{x\left(x-15\right)}{x^2-25}\)

  = \(\dfrac{x\left(x+5\right)}{\left(x-5\right)\left(x+5\right)}+\dfrac{4x\left(x-5\right)}{\left(x+5\right)\left(x-5\right)}+\dfrac{x\left(x-15\right)}{\left(x+5\right)\left(x-5\right)}\)

  = \(\dfrac{x^2+5x+4x^2-20x+x^2-15x}{\left(x-5\right)\left(x+5\right)}\)

  = \(\dfrac{6x^2-30x}{\left(x-5\right)\left(x+5\right)}\)

  = \(\dfrac{6x\left(x-5\right)}{\left(x-5\right)\left(x+5\right)}\)

  = \(\dfrac{6x}{x+5}\)

\(đk:x\ne1\\ \dfrac{x^2+5x}{3x^2-6x+3}:\dfrac{7x+35}{6x-6}\\ =\dfrac{x\left(x+5\right)}{3\left(x^2-2x+1\right)}:\dfrac{7\left(x+5\right)}{6\left(x-1\right)}\\ =\dfrac{x\left(x+5\right)}{3\left(x-1\right)^2}\times\dfrac{6\left(x-1\right)}{7\left(x+5\right)}\\ =\dfrac{2x}{7\left(x-1\right)}\)

\(đk:x\ne1\)

\(\dfrac{x^2+5}{3x^2-6x+3}.\dfrac{7x+35}{6x-6}\\ =\dfrac{x^2+5}{3\left(x^2-2x+1\right)}.\dfrac{7\left(x+5\right)}{6\left(x-1\right)}\\ =\dfrac{x^2+5}{3\left(x-1\right)^2}.\dfrac{7\left(x+5\right)}{6\left(x-1\right)}\\ =\dfrac{7\left(x^2+5\right)\left(x+5\right)}{18.\left(x-1\right)^3}\)

\(đk:x\ne0;1;-1\\ \dfrac{x^2+x}{x^2-x}.\dfrac{2x^2-2x}{x+1}\\ =\dfrac{x\left(x+1\right)}{x\left(x-1\right)}.\dfrac{2x\left(x-1\right)}{x+1}\\ =2x\)

\(đk:x\ne1\\ \dfrac{3x}{x-1}-\dfrac{5x+1}{2x-2}\\ =\dfrac{3x}{x-1}-\dfrac{5x+1}{2\left(x-1\right)}\\ =\dfrac{2.3x}{2\left(x-1\right)}-\dfrac{5x+1}{2\left(x-1\right)}\\ =\dfrac{6x-5x-1}{2\left(x-1\right)}\\ =\dfrac{x-1}{2\left(x-1\right)}\\ =\dfrac{1}{2}\)

\(5x^2+5y^2+8xy-2x+2y+2=0\)

\(\Leftrightarrow4\left(x+y\right)^2+\left(x-1\right)^2+\left(y+1\right)^2=0\)

Vì \(\left(x+y\right)^2\ge0,\left(x-1\right)^2\ge0,\left(y+1\right)^2\ge0\)

\(\Rightarrow4\left(x+y\right)^2+\left(x-1\right)^2+\left(y+1\right)^2\ge0\)

Dấu "=" xảy ra khi \(\left\{{}\begin{matrix}x+y=0\\x-1=0\\y+1=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=-1\end{matrix}\right.\)

\(\left(x+y\right)^{2018}+\left(x-2\right)^{2019}+\left(y+1\right)^{2020}=\left(1-1\right)^{2018}+\left(1-2\right)^{2019}+\left(-1+1\right)^{2020}=-1\)

\(a,đk\left(B\right):x\ne\pm3\\ B=\dfrac{3}{x-3}-\dfrac{6x}{9-x^2}+\dfrac{x}{x+3}\\ =\dfrac{3}{x-3}+\dfrac{6x}{x^2-9}+\dfrac{x}{x+3}\\ =\dfrac{3\left(x+3\right)+6x+x\left(x-3\right)}{x^2-9}\\ =\dfrac{3x+9+6x+x^2-3x}{x^2-9}\\ =\dfrac{x^2+6x+9}{x^2-9}\\ =\dfrac{\left(x+3\right)^2}{x^2-9}\\ =\dfrac{x+3}{x-3}\)

\(b,P=A.B\\ =\dfrac{x+1}{x+3}\times\dfrac{x+3}{x-3}\\ =\dfrac{x+1}{x-3}\)

\(c,\) Để P nguyên 

\(\dfrac{x+1}{x-3}=1+\dfrac{4}{x-3}\)

=> \(x-3\inƯ\left(4\right)\)

\(Ư\left(4\right)=\left\{-1;1;2;-2;4;-4\right\}\)

\(=>x=\left\{2;4;5;1;7;-1\right\}\)

đề bài hỏi cái gì á bạn