Question 90921
you can build a perfect square trinomial from scratch ... (m+n)^2 ... FOILing gives m^2+mn+mn+n^2 or m^2+2mn+n^2


in your problem, m is 4x and n is -5 ... (4x-5)^2=(4x)^2+4x(-5)+4x(-5)+(-5)^2=16x^2-20x-20x+25=16x^2-40x+25


the example doesn't have the minus sign associated with the 5, which could be causing confusion