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-3x^{2}-9x+36=0
All equations of the form ax^{2}+bx+c=0 can be solved using the quadratic formula: \frac{-b±\sqrt{b^{2}-4ac}}{2a}. The quadratic formula gives two solutions, one when ± is addition and one when it is subtraction.
x=\frac{-\left(-9\right)±\sqrt{\left(-9\right)^{2}-4\left(-3\right)\times 36}}{2\left(-3\right)}
This equation is in standard form: ax^{2}+bx+c=0. Substitute -3 for a, -9 for b, and 36 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.
x=\frac{-\left(-9\right)±\sqrt{81-4\left(-3\right)\times 36}}{2\left(-3\right)}
Square -9.
x=\frac{-\left(-9\right)±\sqrt{81+12\times 36}}{2\left(-3\right)}
Multiply -4 times -3.
x=\frac{-\left(-9\right)±\sqrt{81+432}}{2\left(-3\right)}
Multiply 12 times 36.
x=\frac{-\left(-9\right)±\sqrt{513}}{2\left(-3\right)}
Add 81 to 432.
x=\frac{-\left(-9\right)±3\sqrt{57}}{2\left(-3\right)}
Take the square root of 513.
x=\frac{9±3\sqrt{57}}{2\left(-3\right)}
The opposite of -9 is 9.
x=\frac{9±3\sqrt{57}}{-6}
Multiply 2 times -3.
x=\frac{3\sqrt{57}+9}{-6}
Now solve the equation x=\frac{9±3\sqrt{57}}{-6} when ± is plus. Add 9 to 3\sqrt{57}.
x=\frac{-\sqrt{57}-3}{2}
Divide 9+3\sqrt{57} by -6.
x=\frac{9-3\sqrt{57}}{-6}
Now solve the equation x=\frac{9±3\sqrt{57}}{-6} when ± is minus. Subtract 3\sqrt{57} from 9.
x=\frac{\sqrt{57}-3}{2}
Divide 9-3\sqrt{57} by -6.
x=\frac{-\sqrt{57}-3}{2} x=\frac{\sqrt{57}-3}{2}
The equation is now solved.
-3x^{2}-9x+36=0
Quadratic equations such as this one can be solved by completing the square. In order to complete the square, the equation must first be in the form x^{2}+bx=c.
-3x^{2}-9x+36-36=-36
Subtract 36 from both sides of the equation.
-3x^{2}-9x=-36
Subtracting 36 from itself leaves 0.
\frac{-3x^{2}-9x}{-3}=-\frac{36}{-3}
Divide both sides by -3.
x^{2}+\left(-\frac{9}{-3}\right)x=-\frac{36}{-3}
Dividing by -3 undoes the multiplication by -3.
x^{2}+3x=-\frac{36}{-3}
Divide -9 by -3.
x^{2}+3x=12
Divide -36 by -3.
x^{2}+3x+\left(\frac{3}{2}\right)^{2}=12+\left(\frac{3}{2}\right)^{2}
Divide 3, the coefficient of the x term, by 2 to get \frac{3}{2}. Then add the square of \frac{3}{2} to both sides of the equation. This step makes the left hand side of the equation a perfect square.
x^{2}+3x+\frac{9}{4}=12+\frac{9}{4}
Square \frac{3}{2} by squaring both the numerator and the denominator of the fraction.
x^{2}+3x+\frac{9}{4}=\frac{57}{4}
Add 12 to \frac{9}{4}.
\left(x+\frac{3}{2}\right)^{2}=\frac{57}{4}
Factor x^{2}+3x+\frac{9}{4}. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.
\sqrt{\left(x+\frac{3}{2}\right)^{2}}=\sqrt{\frac{57}{4}}
Take the square root of both sides of the equation.
x+\frac{3}{2}=\frac{\sqrt{57}}{2} x+\frac{3}{2}=-\frac{\sqrt{57}}{2}
Simplify.
x=\frac{\sqrt{57}-3}{2} x=\frac{-\sqrt{57}-3}{2}
Subtract \frac{3}{2} from both sides of the equation.