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Which statement describes the first step to solve the equation by completing the square? 3x^2 + 18x= 21

A. Multiply both sides of the equation by 19.
B. Add 81 to each side of the equation.
C. Add 9 to each side of the equation.(MY ANSWER)
D. Multiply both sides of the equation by 13 .

Respuesta :

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Solve for x over the real numbers by completing the Square: None of the provided answers are correct.
3 x^2 + 18 x = 21

Subtract 21 from both sides:
3 x^2 + 18 x - 21 = 0

3 x^2 + 18 x - 21 = 3 (x^2 + 6 x - 7):
3 (x^2 + 6 x - 7) = 0

Divide both sides by 3:
x^2 + 6 x - 7 = 0

x = (-6 ± sqrt(6^2 - 4 (-7)))/2 = (-6 ± sqrt(36 + 28))/2 = (-6 ± sqrt(64))/2:
x = (-6 + sqrt(64))/2 or x = (-6 - sqrt(64))/2

sqrt(64) = sqrt(2^6) = 8 = 8:
x = (-6 + 8)/2 or x = (-6 - 8)/2

(-6 + 8)/2 = 2/2 = 1:
x = 1 or x = (-6 - 8)/2

(-6 - 8)/2 = -14/2 = -7:

Answer:  x = 1 or x = -7

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