Solve sqrt{y+4}=y

$$\sqrt { y + 4 } = y$$

Solve for y

$$y = \frac{\sqrt{17} + 1}{2} \approx 2.561552813$$

Solution Steps

Square both sides of the equation.

$$\left(\sqrt{y+4}\right)^{2}=y^{2}$$

Calculate \sqrt{y+4} to the power of 2 and get y+4.

$$y+4=y^{2}$$

Subtract y^{2} from both sides.

$$y+4-y^{2}=0$$

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.

$$-y^{2}+y+4=0$$

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

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

Square 1.

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

Multiply -4 times -1.

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

Multiply 4 times 4.

$$y=\frac{-1±\sqrt{1+16}}{2\left(-1\right)}$$

Add 1 to 16.

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

Multiply 2 times -1.

$$y=\frac{-1±\sqrt{17}}{-2}$$

Now solve the equation y=\frac{-1±\sqrt{17}}{-2} when ± is plus. Add -1 to \sqrt{17}.

$$y=\frac{\sqrt{17}-1}{-2}$$

Divide -1+\sqrt{17} by -2.

$$y=\frac{1-\sqrt{17}}{2}$$

Now solve the equation y=\frac{-1±\sqrt{17}}{-2} when ± is minus. Subtract \sqrt{17} from -1.

$$y=\frac{-\sqrt{17}-1}{-2}$$

Divide -1-\sqrt{17} by -2.

$$y=\frac{\sqrt{17}+1}{2}$$

The equation is now solved.

$$y=\frac{1-\sqrt{17}}{2} y=\frac{\sqrt{17}+1}{2}$$

Substitute \frac{1-\sqrt{17}}{2} for y in the equation \sqrt{y+4}=y.

$$\sqrt{\frac{1-\sqrt{17}}{2}+4}=\frac{1-\sqrt{17}}{2}$$

Simplify. The value y=\frac{1-\sqrt{17}}{2} does not satisfy the equation because the left and the right hand side have opposite signs.

$$-\left(\frac{1}{2}-\frac{1}{2}\times 17^{\frac{1}{2}}\right)=\frac{1}{2}-\frac{1}{2}\times 17^{\frac{1}{2}}$$

Substitute \frac{\sqrt{17}+1}{2} for y in the equation \sqrt{y+4}=y.

$$\sqrt{\frac{\sqrt{17}+1}{2}+4}=\frac{\sqrt{17}+1}{2}$$

Simplify. The value y=\frac{\sqrt{17}+1}{2} satisfies the equation.

$$\frac{1}{2}+\frac{1}{2}\times 17^{\frac{1}{2}}=\frac{1}{2}\times 17^{\frac{1}{2}}+\frac{1}{2}$$

Equation \sqrt{y+4}=y has a unique solution.

$$y=\frac{\sqrt{17}+1}{2}$$