X^2+12=40 what number must you add to complete the square?

Step-by-step explanation:

1) If you really meant X^2+12=40, then this simplifies to x^2 = 28, and therefore x = ±√28, or x = ±2√7.

2) If you meant X^2+12x=40:

a) Take half of the coefficient of x:  that would be (1/2)(12), or 6.  

b) Square this result, obtaining:  6² = 36

c) Add this 36 to x^2 + 12x + 40, and then subtract it:  We get:  

x² + 12x + 36 - 36 = 40, or x² + 12x + 36 = 76

We have to add 36, as indicated above, to "complete the square."