Solve -y^2+10y+400=0

$$- { y }^{ 2 } +10y+400=0$$

Solve for y

$$y=5\sqrt{17}+5\approx 25.615528128$$

$$y=5-5\sqrt{17}\approx -15.615528128$$

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.

$$-y^{2}+10y+400=0$$

This equation is in standard form: ax^{2}+bx+c=0. Substitute -1 for a, 10 for b, and 400 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.

$$y=\frac{-10±\sqrt{10^{2}-4\left(-1\right)\times 400}}{2\left(-1\right)}$$

Square 10.

$$y=\frac{-10±\sqrt{100-4\left(-1\right)\times 400}}{2\left(-1\right)}$$

Multiply -4 times -1.

$$y=\frac{-10±\sqrt{100+4\times 400}}{2\left(-1\right)}$$

Multiply 4 times 400.

$$y=\frac{-10±\sqrt{100+1600}}{2\left(-1\right)}$$

Add 100 to 1600.

$$y=\frac{-10±\sqrt{1700}}{2\left(-1\right)}$$

Take the square root of 1700.

$$y=\frac{-10±10\sqrt{17}}{2\left(-1\right)}$$

Multiply 2 times -1.

$$y=\frac{-10±10\sqrt{17}}{-2}$$

Now solve the equation y=\frac{-10±10\sqrt{17}}{-2} when ± is plus. Add -10 to 10\sqrt{17}.

$$y=\frac{10\sqrt{17}-10}{-2}$$

Divide -10+10\sqrt{17} by -2.

$$y=5-5\sqrt{17}$$

Now solve the equation y=\frac{-10±10\sqrt{17}}{-2} when ± is minus. Subtract 10\sqrt{17} from -10.

$$y=\frac{-10\sqrt{17}-10}{-2}$$

Divide -10-10\sqrt{17} by -2.

$$y=5\sqrt{17}+5$$

The equation is now solved.

$$y=5-5\sqrt{17} y=5\sqrt{17}+5$$

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.

$$-y^{2}+10y+400=0$$

Subtract 400 from both sides of the equation.

$$-y^{2}+10y+400-400=-400$$

Subtracting 400 from itself leaves 0.

$$-y^{2}+10y=-400$$

Divide both sides by -1.

$$\frac{-y^{2}+10y}{-1}=-\frac{400}{-1}$$

Dividing by -1 undoes the multiplication by -1.

$$y^{2}+\frac{10}{-1}y=-\frac{400}{-1}$$

Divide 10 by -1.

$$y^{2}-10y=-\frac{400}{-1}$$

Divide -400 by -1.

$$y^{2}-10y=400$$

Divide -10, the coefficient of the x term, by 2 to get -5. Then add the square of -5 to both sides of the equation. This step makes the left hand side of the equation a perfect square.

$$y^{2}-10y+\left(-5\right)^{2}=400+\left(-5\right)^{2}$$

Square -5.

$$y^{2}-10y+25=400+25$$

Add 400 to 25.

$$y^{2}-10y+25=425$$

Factor y^{2}-10y+25. In general, when x^{2}+bx+c is a perfect square, it can always be factored as \left(x+\frac{b}{2}\right)^{2}.

$$\left(y-5\right)^{2}=425$$

Take the square root of both sides of the equation.

$$\sqrt{\left(y-5\right)^{2}}=\sqrt{425}$$

Simplify.

$$y-5=5\sqrt{17} y-5=-5\sqrt{17}$$

Add 5 to both sides of the equation.

$$y=5\sqrt{17}+5 y=5-5\sqrt{17}$$