Question 783793
What is the meaning of A and B?  
You give an equation and you give a mention about A and B but nowhere indicate what these two variables mean.  


A best-guess is that the A and B are part of the more general, {{{Ax^2+Bx+C=0 }}}quadratic equation.  You do NOT make A=1.  In your given equation, {{{5x^2+25x+10=45}}}, A=5.  Also, B=25.  You can Complete the Square if you like, but A=5 and B=25 whatever you do.  


What you want when you Complete the Square is to factorize the quadratic expression in your equation which effectively gives you a quadratic expression in which the coefficient on {{{x^2}}} will be 1.  Your left side would go like this:

{{{5x^2+25x+10}}}
{{{5(x^2+5x+2)}}}
and you would determine that the square term to add & subtract, or to just add to both sides of the equation, would be {{{(5/2)^2=25/4}}},  which corresponds to the more general {{{(B/2)^2=(1/4)B^2}}}.



Note, what I should also say,
WITHIN the quadratic factor, {{{x^2+5x+2}}}, A=1 and B=5, but this is only if we first ignore labeling the coefficients in the original equation and focus only on the coefficients in the quadratic factor.  To say that A=1 and A=5 is really a BAD IDEA.