Question 141902
To make the trinomials perfect square trinomials, you must add the square of half the x-coeffecient, as long as the x^2-coefficient is 1.

{{{x^2+11x}}} The {{{x^2}}}-coefficient is 1 so add the square of half the x-coefficient.  Add {{{(11/2)^2 = 121/4}}} to get:
{{{x^2+11x+121/4}}}
Check:
{{{x^2+11x+121/4 = (x+11/2)(x+11/2)}}}={{{(x+11/2)^2}}}
Likewise for:
{{{x^2+(4/5)x}}} Add {{{((1/2)(4/5))^2 = (4/10)^2}}} = {{{16/100 = 4/25}}} to get:
{{{x^2+(4/5)x+4/25}}}