Solve 5y^2-90y+54=0

$$5 { y }^{ 2 } -90y+54 = 0$$

Solve for y

$$y=\frac{3\sqrt{195}}{5}+9\approx 17.378544026$$

$$y=-\frac{3\sqrt{195}}{5}+9\approx 0.621455974$$

Steps Using the Quadratic Formula

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.

$$5y^{2}-90y+54=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 5 for a, -90 for b, and 54 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$y=\frac{-\left(-90\right)±\sqrt{\left(-90\right)^{2}-4\times 5\times 54}}{2\times 5}$$

Square -90.

$$y=\frac{-\left(-90\right)±\sqrt{8100-4\times 5\times 54}}{2\times 5}$$

Multiply -4 times 5.

$$y=\frac{-\left(-90\right)±\sqrt{8100-20\times 54}}{2\times 5}$$

Multiply -20 times 54.

$$y=\frac{-\left(-90\right)±\sqrt{8100-1080}}{2\times 5}$$

Add 8100 to -1080.

$$y=\frac{-\left(-90\right)±\sqrt{7020}}{2\times 5}$$

Take the square root of 7020.

$$y=\frac{-\left(-90\right)±6\sqrt{195}}{2\times 5}$$

The opposite of -90 is 90.

$$y=\frac{90±6\sqrt{195}}{2\times 5}$$

Multiply 2 times 5.

$$y=\frac{90±6\sqrt{195}}{10}$$

Now solve the equation y=\frac{90±6\sqrt{195}}{10} when ± is plus. Add 90 to 6\sqrt{195}.

$$y=\frac{6\sqrt{195}+90}{10}$$

Divide 90+6\sqrt{195} by 10.

$$y=\frac{3\sqrt{195}}{5}+9$$

Now solve the equation y=\frac{90±6\sqrt{195}}{10} when ± is minus. Subtract 6\sqrt{195} from 90.

$$y=\frac{90-6\sqrt{195}}{10}$$

Divide 90-6\sqrt{195} by 10.

$$y=-\frac{3\sqrt{195}}{5}+9$$

The equation is now solved.

$$y=\frac{3\sqrt{195}}{5}+9 y=-\frac{3\sqrt{195}}{5}+9$$

Steps for Completing the Square

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.

$$5y^{2}-90y+54=0$$

Subtract 54 from both sides of the equation.

$$5y^{2}-90y+54-54=-54$$

Subtracting 54 from itself leaves 0.

$$5y^{2}-90y=-54$$

Divide both sides by 5.

$$\frac{5y^{2}-90y}{5}=-\frac{54}{5}$$

Dividing by 5 undoes the multiplication by 5.

$$y^{2}+\left(-\frac{90}{5}\right)y=-\frac{54}{5}$$

Divide -90 by 5.

$$y^{2}-18y=-\frac{54}{5}$$

Divide -18, the coefficient of the x term, by 2 to get -9. Then add the square of -9 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$y^{2}-18y+\left(-9\right)^{2}=-\frac{54}{5}+\left(-9\right)^{2}$$

Square -9.

$$y^{2}-18y+81=-\frac{54}{5}+81$$

Add -\frac{54}{5} to 81.

$$y^{2}-18y+81=\frac{351}{5}$$

Factor y^{2}-18y+81. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(y-9\right)^{2}=\frac{351}{5}$$

Take the square root of both sides of the equation.

$$\sqrt{\left(y-9\right)^{2}}=\sqrt{\frac{351}{5}}$$

Simplify.

$$y-9=\frac{3\sqrt{195}}{5} y-9=-\frac{3\sqrt{195}}{5}$$

Add 9 to both sides of the equation.

$$y=\frac{3\sqrt{195}}{5}+9 y=-\frac{3\sqrt{195}}{5}+9$$