If using the method of completing the square to solve the quadratic equation x^2+13x+15=0 which number would have to be added to "complete the square"?

The number to be added to complete the square is (13/2)^2

But the solved value for x are:

x = 11.71 or -1.28

Step-by-step explanation:

x ^ 2 + 13x + 15 = 0

x^2 + 13x = -15

(The coefficient of x^2 is 1, hence needless diving through)

Add the square of half the coefficient of x to both sides

x^2 + 13x + (13/2)^2 = -15 + ( 13/2)^2

(x + 13/2)^2 = -15 + 169/4

(x + 13/2)^2 = 109/4

Take the square of both sides

x + 13/2 = +-10.44/ 2

x = 10.44/2 + 13 /2

x = 23.44/2

x = 11.71

OR

x = 10.44/2 - 13/2

x = -2.56/2

x = -1.28

Therefore x = 11.71 or -1.28

But the number to be added to complete the square is (13/2)^2

Lets keep learning, maths is fun

e^-kt = n/n0 = 0.79  

-kt = ln 0.79  

t = - ln 0.79/0.0001  

= 2357 years

step-by-step explanation: