13.
$7a^3-28a=7a(a^2-4)=7a(a^2-2^2)=7a(a-2)(a+2)$
14.
$a^3-10a^2+25a=a(a^2-10a+25)=a(a-5)^2$
15.
$x^3-8+2x(x-2)=(x-2)(x^2+2x+4)+2x(x-2)$
$=(x-2)(x^2+2x+4+2x)$
$=(x-2)(x^2+4x+4)=(x-2)(x+2)^2$
16.
$x^2-25-(5-x)(x+3)=(x^2-5^2)+(x-5)(x+3)$
$=(x-5)(x+5)+(x-5)(x+3)$
$=(x-5)(x+5+x+3)=(x-5)(2x+8)=2(x-5)(x+4)$
13) 7a³ - 28a
= 7a(a² - 4)
= 7a(a - 2)(a + 2)
14) a³ - 10a² + 25a
= a(a² - 10a + 25)
= a(a - 5)²
15) x³ - 8 + 2x(x - 2)
= (x³ - 2³) + 2x(x - 2)
= (x - 2)(x² + 2x + 4) + 2x(x - 2)
= (x - 2)(x² + 2x + 4 + 2x)
= (x - 2)(x² + 4x + 4)
= (x - 2)(x + 2)²
16) x² - 25 - (5 - x)(x + 3)
= (x² - 5²) + (x - 5)(x + 3)
= (x - 5)(x + 5) + (x - 5)(x + 3)
= (x - 5)(x + 5 + x + 3)
= (x - 5)(2x + 8)
= 2(x - 5)(x + 4)
\(13.7a\left(a^{^2}-4\right)=7a\left(a-2\right)\left(a+2\right)\\ 14.a\left(a^{^2}-10a+25\right)=a\left(a-5\right)^{^2}\\ 15.\left(x-2\right)\left(x^{^2}+2x+4\right)+2x\left(x-2\right)\\ =\left(x-2\right)\left(x^{^2}+2x+4+2x\right)\\ =\left(x-2\right)\left(x^{^2}+4x+4\right)\\ =\left(x-2\right)\left(x+2\right)^{^2}\\ 16.\left(x-5\right)\left(x+5\right)-\left(5-x\right)\left(x+3\right)\\ =\left(x-5\right)\left(x+5+x+3\right)\\ =\left(x-5\right)\left(2x+8\right)\\ =2\left(x-5\right)\left(x+4\right)\)
