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a)
\(A=\left(x-6\right)^2+\left(x+6\right)^2\)
\(A=\left(x^2-2x6+6^2\right)+\left(x^2+2x6+6^2\right)\)
\(A=x^2-2x6+6^2+x^2+2x6+6^2\)
\(A=\left(x^2+x^2\right)+\left(-2x6+2x6\right)+\left(6^2+6^2\right)\)
\(A=2x^2+72\)
b)
\(B=\left(x^2+y^2+3^2+2xy+2x3+2y3\right)-\left(x^2+y^2+9\right)\)
\(B=x^2+y^2+3^3+2xy+2x3+2y3-x^2-y^2-9\)
\(B=\left(x^2-x^2\right)+\left(y^2-y^2\right)+\left(3^2-9\right)+2xy+2x3+2y3\)
\(B=2xy+2x3+2y3\)
Mình phải đi ngủ rồi, có gì mai làm tiếp nha 
c/
C = (5x - 2) . (5x + 2) - (5x - 1)2
C = [(5x)2 - 22] - [(5x)2 - 2 . 5x1 + 12]
C = (5x)2 - 22 - (5x)2 + 2 . 5x1 - 12
C = [(5x)2 - (5x)2] + (-22 + 2 - 12) + 5x1
C = 5 + 5x1.
a: \(=\left(2x^2-x\right)^2-5\left(2x^2-x\right)-6\)
\(=\left(2x^2-x\right)^2-6\left(2x^2-x\right)+\left(2x^2-x-6\right)\)
\(=\left(2x^2-x-6\right)\left(2x^2-x+1\right)\)
\(=\left(2x^2-4x+3x-6\right)\left(2x^2-x+1\right)\)
\(=\left(x-2\right)\left(2x+3\right)\left(2x^2-x+1\right)\)
b: \(=\left(5x^2-2x\right)^2-\left(5x^2-2x\right)-6\)
\(=\left(5x^2-2x-6\right)\left(5x^2-2x+1\right)\)
d: \(=\left(x+2\right)\left(x-1\right)\left(x+3\right)\left(x+6\right)-28\)
\(=\left(x^2+5x+6\right)\left(x^2+5x-6\right)-28\)
\(=\left(x^2+5x\right)^2-36-28\)
\(=\left(x^2+5x\right)^2-64\)
\(=\left(x^2+5x+8\right)\left(x^2+5x-8\right)\)
a) \(\left(3x+2\right)^2-\left(3x-2\right)^2=5x+8\)
\(\Rightarrow\left(3x+2+3x-2\right)\left(3x+2-3x+2\right)=5x+8\)
\(\Rightarrow4.6x=5x+8\Rightarrow24x=5x+8\)
\(\Rightarrow19x=8\Rightarrow x=\frac{8}{19}\)
b) \(3\left(x-2\right)^2+9\left(x-1\right)=3\left(x^2+x-3\right)\)
\(\Rightarrow3\left(x^2-4x+4\right)+9x-9=3x^2+3x-9\)
\(\Rightarrow3x^2-12x+12+9x-9=3x^2+3x-9\)
\(\Rightarrow-12x+12+9x-9=3x-9\)
\(\Rightarrow-3x+3=3x-9\)
\(\Rightarrow6x=12\Rightarrow x=2\)
a) \(\dfrac{2}{3x+9}-\dfrac{x-3}{3x^2+9x}\)
\(=\dfrac{2}{3\left(x+3\right)}-\dfrac{x-3}{3x\left(x+3\right)}\)
\(=\dfrac{2x}{3x\left(x+3\right)}-\dfrac{x-3}{3x\left(x+3\right)}\)
\(=\dfrac{2x-x+3}{3x\left(x+3\right)}\)
\(=\dfrac{x+3}{3x\left(x+3\right)}\)
\(=\dfrac{1}{3x}\)
b) \(\dfrac{x^2+x}{5x^2-10x+5}:\dfrac{3x+3}{5x-5}\)
\(=\dfrac{x\left(x+1\right)}{5\left(x^2-2x+1\right)}:\dfrac{3\left(x+1\right)}{5\left(x-1\right)}\)
\(=\dfrac{x\left(x+1\right)}{5\left(x-1\right)^2}:\dfrac{3\left(x+1\right)}{5\left(x-1\right)}\)
\(=\dfrac{x\left(x+1\right)}{5\left(x-1\right)^2}.\dfrac{5\left(x-1\right)}{3\left(x+1\right)}\)
\(=\dfrac{x}{\left(x-1\right).3}\)
\(=\dfrac{x}{3x-3}\)
c) \(\dfrac{1}{x\left(x+1\right)}+\dfrac{1}{\left(x+1\right)\left(x+2\right)}+...+\dfrac{1}{\left(x+99\right)\left(x+100\right)}\)
\(=\dfrac{1}{x}-\dfrac{1}{x+1}+\dfrac{1}{x+1}-\dfrac{1}{x+2}+...+\dfrac{1}{x+99}-\dfrac{1}{x+100}\)
\(=\dfrac{1}{x}-\dfrac{1}{x+100}\)
\(=\dfrac{x+100}{x\left(x+100\right)}-\dfrac{x}{x\left(x+100\right)}\)
\(=\dfrac{x+100-x}{x\left(x+100\right)}\)
\(=\dfrac{100}{x\left(x+100\right)}\)
Ta có (5x-11)2+(4-5x)2+2(5x-11)(4-5x)
=25x2-110x+121+16-40x+25x2+40x-50x2- 88+110x
=16-88+121
=49(đpcm)
TK MK NHA BN
\(B=\left(5x-11\right)^2-\left(10x-22\right)\left(5x-9\right)+\left(5x-9\right)^2\)
\(=\left(5x-11\right)^2-2\left(5x-11\right)\left(5x-9\right)+\left(5x-9\right)^2\)
\(=\left[\left(5x-11\right)-\left(5x-9\right)\right]^2\)
\(=\left(-2\right)^2=4\)