Solve y^2-30y+210=0

$${ y }^{ 2 } -30y+210 = 0$$

Solve for y

$$y=\sqrt{15}+15\approx 18.872983346$$

$$y=15-\sqrt{15}\approx 11.127016654$$

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.

$$y^{2}-30y+210=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -30 for b, and 210 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$y=\frac{-\left(-30\right)±\sqrt{\left(-30\right)^{2}-4\times 210}}{2}$$

Square -30.

$$y=\frac{-\left(-30\right)±\sqrt{900-4\times 210}}{2}$$

Multiply -4 times 210.

$$y=\frac{-\left(-30\right)±\sqrt{900-840}}{2}$$

Add 900 to -840.

$$y=\frac{-\left(-30\right)±\sqrt{60}}{2}$$

Take the square root of 60.

$$y=\frac{-\left(-30\right)±2\sqrt{15}}{2}$$

The opposite of -30 is 30.

$$y=\frac{30±2\sqrt{15}}{2}$$

Now solve the equation y=\frac{30±2\sqrt{15}}{2} when ± is plus. Add 30 to 2\sqrt{15}.

$$y=\frac{2\sqrt{15}+30}{2}$$

Divide 30+2\sqrt{15} by 2.

$$y=\sqrt{15}+15$$

Now solve the equation y=\frac{30±2\sqrt{15}}{2} when ± is minus. Subtract 2\sqrt{15} from 30.

$$y=\frac{30-2\sqrt{15}}{2}$$

Divide 30-2\sqrt{15} by 2.

$$y=15-\sqrt{15}$$

The equation is now solved.

$$y=\sqrt{15}+15 y=15-\sqrt{15}$$

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.

$$y^{2}-30y+210=0$$

Subtract 210 from both sides of the equation.

$$y^{2}-30y+210-210=-210$$

Subtracting 210 from itself leaves 0.

$$y^{2}-30y=-210$$

Divide -30, the coefficient of the x term, by 2 to get -15. Then add the square of -15 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$y^{2}-30y+\left(-15\right)^{2}=-210+\left(-15\right)^{2}$$

Square -15.

$$y^{2}-30y+225=-210+225$$

Add -210 to 225.

$$y^{2}-30y+225=15$$

Factor y^{2}-30y+225. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(y-15\right)^{2}=15$$

Take the square root of both sides of the equation.

$$\sqrt{\left(y-15\right)^{2}}=\sqrt{15}$$

Simplify.

$$y-15=\sqrt{15} y-15=-\sqrt{15}$$

Add 15 to both sides of the equation.

$$y=\sqrt{15}+15 y=15-\sqrt{15}$$