Explanation:
completing the square means finding an expression
(x + a)² = b²
or
(x - a)² = b²
(x + a)² = x² + 2ax + a²
(x - a)² = x² - 2ax + a²
because of the "+ 18x" in the original equation we know that we are looking for (x + a)².
now, let's compare the different terms :
x² = x²
18x = 2ax
18 = 2a
a = 9
so, the complete square would look like
(x + 9)² = x² + 18x + 81
but we have only "+ 79" in the original equation.
so, we add the missing 2 (to both sides to keep the equation true)
x² + 18x + 79 + 2 = 0 + 2
x² + 18x + 81 = 2
(x + 9)² = 2
x + 9 = ±sqrt(2)
x1 = sqrt(2) - 9
x2 = -sqrt(2) - 9
x = sqrt(2) - 9, -sqrt(2) - 9
yes, every quadratic equation has 2 solutions.
as every cubic equation has 3 solutions.
and so on.