1. ## factorising polynomials

Hi does anyone know how to factorise 80 - (5k)^2

You have $(\sqrt{80})^2 - (5k)^2$ so use the Difference of Two Squares formula $A^2 - B^2 = (A - B)(A + B)$.
$80-(5k)^{2}=80-25k^{2}=-5(5k^2-16)$