x^2+8x+21=0

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Solution for x^2+8x+21=0 equation:


Simplifying
x2 + 8x + 21 = 0

Reorder the terms:
21 + 8x + x2 = 0

Solving
21 + 8x + x2 = 0

Solving for variable 'x'.

Begin completing the square.

Move the constant term to the right:

Add '-21' to each side of the equation.
21 + 8x + -21 + x2 = 0 + -21

Reorder the terms:
21 + -21 + 8x + x2 = 0 + -21

Combine like terms: 21 + -21 = 0
0 + 8x + x2 = 0 + -21
8x + x2 = 0 + -21

Combine like terms: 0 + -21 = -21
8x + x2 = -21

The x term is 8x.  Take half its coefficient (4).
Square it (16) and add it to both sides.

Add '16' to each side of the equation.
8x + 16 + x2 = -21 + 16

Reorder the terms:
16 + 8x + x2 = -21 + 16

Combine like terms: -21 + 16 = -5
16 + 8x + x2 = -5

Factor a perfect square on the left side:
(x + 4)(x + 4) = -5

Can't calculate square root of the right side.

The solution to this equation could not be determined.

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