If you are solving y 2+2y=48 by completing the square, the next line would be y 2 + 2y - 48 = 0 y 2 + 2y + 4 = 52 y 2 + 2y + 1 = 49

so let's start off by grouping y²+2y=48

(y² + 2y +[?]²) = 48

now, we have a missing value to get a perfect square trinomial.

recall that the middle term is the product of 2 and the other two without the ², so one can say that

low and behold, is 1, now, since all we're doing is borrowing from our very good friend Mr Zero, 0, if we add 1², we also have to subtract 1².

goog may have it for you