a) Ta có : (x - 5)2 - 16
= (x - 5)2 - 42
= (x - 5 - 4)(x - 5 + 4)
= (x - 1)(x - 9)
b) 25 - (3 - x)2
= 52 - (3 - x)2
= (5 - 3 + x)(5 + 3 - x)
= (x + 2)(8 - x)
c) (7x - 4)2 - (2x + 1)2
= (7x - 4 - 2x - 1)(7x - 4 + 2x + 1)
= (5x - 5)(9x - 3)
= 5(x - 1)3(3x - 1)
= 15(x - 1)(3x - 1)
\(49.\left(y-4\right)^2-9\left(y+2\right)^2\)
\(=\left[7.\left(y-4\right)\right]^2-\left[3.\left(y+2\right)\right]^2\)
\(=\left[7.\left(y-4\right)-3.\left(y+2\right)\right].\left[7.\left(y-4\right)+3.\left(y+2\right)\right]\)
\(=\left(7y-28-3y-6\right).\left(7y-28+3y+6\right)\)
\(=\left(4y-34\right).\left(10y-22\right)\)
\(=4.\left(y-17\right).\left(5y-11\right)\)
\(8x^3+\frac{1}{27}=\left(2x\right)^3+\left(\frac{1}{3}\right)^3=\left(2x+\frac{1}{3}\right).\left(4x^2+\frac{2}{3}x+\frac{1}{9}\right)\)
\(125-x^6=5^3-\left(x^2\right)^3=\left(5-x^2\right).\left(25+5x^2+x^4\right)=\left(\sqrt{5}-x\right).\left(\sqrt{5}+x\right).\left(25+5x^2+x^4\right)\)