What number must be added to complete thesquare of x2 4x5?

Answer 1

There are missing + or - symbols, but the square is completed in this case by dividing the coefficient of the x term by 2, giving:

(x ± 2)2 = x2 ±4x + 4

So to get to the original equation, there is needed an extra +1 or -9:

(x ± 2)2 +1 = x2 ±4x +5

(x ± 2)2 - 9 = x2 ±4x - 5

Depending upon what the missing (+-) symbols are. (Whatever the sign before the 4x, the same is used in the brackets of the square.)