Question

Solve the equation by completing the square. Round to the nearest hundredth x^2 + 2x = 15

Answer 1

x = 3, x = -5

Explanation:

A perfect square trinomial is represented in the form a^2 + 2ab + b^2. We are already given the a^2 term, x^2, and the 2ab term, 2x. From this we can say:

a^2 = x^2

a = x

Now, we can substitute x for a in the other expression to create the equation:

2ab = 2x

2(x)b=2x

b = 1

From this, b^2 is one, so, to get our trinomial all on one side, we add 1 to both sides:

x^2 + 2x = 15

x^2 + 2x + 1 = 16

Now, we can factor. The perfect square trinomial factors into (a + b)^2. In this case, a is x, and b is one. We can factor and get:

(x + 1)^2 = 16

Now, we take the square root of both sides:

x + 1 = ± 4

We can separate this into two equations and solve:

x + 1 = 4

x = 3

x + 1 = -4

x = -5

Answer 2

Explanation:

x^2 + 2x = 15

x^2 + 2x + [1/2(2)]^2 = 15 + [1/2(2)]^2

(x + 1/2(2) )^2 = 15 + [(1/2)(2)]^2

(x + 1)^2 = 15 + 1^2

(x + 1)^2 = 15 + 1

(x+1)^2 = 16 Take the square root of both sides.

sqrt( (x + 1)^2 ) = sqrt(16)

x + 1 = +/- 4

x + 1 = 4

x = 4 - 1 = 3

x + 1 = -4

x = -4 - 1

x = - 5

So the roots are 3 and - 5