Solve for r
r=\frac{\sqrt{17}+1}{8}\approx 0.640388203
r=\frac{1-\sqrt{17}}{8}\approx -0.390388203
Share
Copied to clipboard
r^{2}-\frac{1}{4}r-\frac{1}{4}=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.
r=\frac{-\left(-\frac{1}{4}\right)±\sqrt{\left(-\frac{1}{4}\right)^{2}-4\left(-\frac{1}{4}\right)}}{2}
This equation is in standard form: ax^{2}+bx+c=0. Substitute 1 for a, -\frac{1}{4} for b, and -\frac{1}{4} for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.
r=\frac{-\left(-\frac{1}{4}\right)±\sqrt{\frac{1}{16}-4\left(-\frac{1}{4}\right)}}{2}
Square -\frac{1}{4} by squaring both the numerator and the denominator of the fraction.
r=\frac{-\left(-\frac{1}{4}\right)±\sqrt{\frac{1}{16}+1}}{2}
Multiply -4 times -\frac{1}{4}.
r=\frac{-\left(-\frac{1}{4}\right)±\sqrt{\frac{17}{16}}}{2}
Add \frac{1}{16} to 1.
r=\frac{-\left(-\frac{1}{4}\right)±\frac{\sqrt{17}}{4}}{2}
Take the square root of \frac{17}{16}.
r=\frac{\frac{1}{4}±\frac{\sqrt{17}}{4}}{2}
The opposite of -\frac{1}{4} is \frac{1}{4}.
r=\frac{\sqrt{17}+1}{2\times 4}
Now solve the equation r=\frac{\frac{1}{4}±\frac{\sqrt{17}}{4}}{2} when ± is plus. Add \frac{1}{4} to \frac{\sqrt{17}}{4}.
r=\frac{\sqrt{17}+1}{8}
Divide \frac{1+\sqrt{17}}{4} by 2.
r=\frac{1-\sqrt{17}}{2\times 4}
Now solve the equation r=\frac{\frac{1}{4}±\frac{\sqrt{17}}{4}}{2} when ± is minus. Subtract \frac{\sqrt{17}}{4} from \frac{1}{4}.
r=\frac{1-\sqrt{17}}{8}
Divide \frac{1-\sqrt{17}}{4} by 2.
r=\frac{\sqrt{17}+1}{8} r=\frac{1-\sqrt{17}}{8}
The equation is now solved.
r^{2}-\frac{1}{4}r-\frac{1}{4}=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.
r^{2}-\frac{1}{4}r-\frac{1}{4}-\left(-\frac{1}{4}\right)=-\left(-\frac{1}{4}\right)
Add \frac{1}{4} to both sides of the equation.
r^{2}-\frac{1}{4}r=-\left(-\frac{1}{4}\right)
Subtracting -\frac{1}{4} from itself leaves 0.
r^{2}-\frac{1}{4}r=\frac{1}{4}
Subtract -\frac{1}{4} from 0.
r^{2}-\frac{1}{4}r+\left(-\frac{1}{8}\right)^{2}=\frac{1}{4}+\left(-\frac{1}{8}\right)^{2}
Divide -\frac{1}{4}, the coefficient of the x term, by 2 to get -\frac{1}{8}. Then add the square of -\frac{1}{8} to both sides of the equation. This step makes the left hand side of the equation a perfect square.
r^{2}-\frac{1}{4}r+\frac{1}{64}=\frac{1}{4}+\frac{1}{64}
Square -\frac{1}{8} by squaring both the numerator and the denominator of the fraction.
r^{2}-\frac{1}{4}r+\frac{1}{64}=\frac{17}{64}
Add \frac{1}{4} to \frac{1}{64} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.
\left(r-\frac{1}{8}\right)^{2}=\frac{17}{64}
Factor r^{2}-\frac{1}{4}r+\frac{1}{64}. 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(r-\frac{1}{8}\right)^{2}}=\sqrt{\frac{17}{64}}
Take the square root of both sides of the equation.
r-\frac{1}{8}=\frac{\sqrt{17}}{8} r-\frac{1}{8}=-\frac{\sqrt{17}}{8}
Simplify.
r=\frac{\sqrt{17}+1}{8} r=\frac{1-\sqrt{17}}{8}
Add \frac{1}{8} to both sides of the equation.
Examples
Quadratic equation
{ x } ^ { 2 } - 4 x - 5 = 0
Trigonometry
4 \sin \theta \cos \theta = 2 \sin \theta
Linear equation
y = 3x + 4
Arithmetic
699 * 533
Matrix
\left[ \begin{array} { l l } { 2 } & { 3 } \\ { 5 } & { 4 } \end{array} \right] \left[ \begin{array} { l l l } { 2 } & { 0 } & { 3 } \\ { -1 } & { 1 } & { 5 } \end{array} \right]
Simultaneous equation
\left. \begin{cases} { 8x+2y = 46 } \\ { 7x+3y = 47 } \end{cases} \right.
Differentiation
\frac { d } { d x } \frac { ( 3 x ^ { 2 } - 2 ) } { ( x - 5 ) }
Integration
\int _ { 0 } ^ { 1 } x e ^ { - x ^ { 2 } } d x
Limits
\lim _{x \rightarrow-3} \frac{x^{2}-9}{x^{2}+2 x-3}