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4y^{2}-28y=16
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.
4y^{2}-28y-16=16-16
Subtract 16 from both sides of the equation.
4y^{2}-28y-16=0
Subtracting 16 from itself leaves 0.
y=\frac{-\left(-28\right)±\sqrt{\left(-28\right)^{2}-4\times 4\left(-16\right)}}{2\times 4}
This equation is in standard form: ax^{2}+bx+c=0. Substitute 4 for a, -28 for b, and -16 for c in the quadratic formula, \frac{-b±\sqrt{b^{2}-4ac}}{2a}.
y=\frac{-\left(-28\right)±\sqrt{784-4\times 4\left(-16\right)}}{2\times 4}
Square -28.
y=\frac{-\left(-28\right)±\sqrt{784-16\left(-16\right)}}{2\times 4}
Multiply -4 times 4.
y=\frac{-\left(-28\right)±\sqrt{784+256}}{2\times 4}
Multiply -16 times -16.
y=\frac{-\left(-28\right)±\sqrt{1040}}{2\times 4}
Add 784 to 256.
y=\frac{-\left(-28\right)±4\sqrt{65}}{2\times 4}
Take the square root of 1040.
y=\frac{28±4\sqrt{65}}{2\times 4}
The opposite of -28 is 28.
y=\frac{28±4\sqrt{65}}{8}
Multiply 2 times 4.
y=\frac{4\sqrt{65}+28}{8}
Now solve the equation y=\frac{28±4\sqrt{65}}{8} when ± is plus. Add 28 to 4\sqrt{65}.
y=\frac{\sqrt{65}+7}{2}
Divide 28+4\sqrt{65} by 8.
y=\frac{28-4\sqrt{65}}{8}
Now solve the equation y=\frac{28±4\sqrt{65}}{8} when ± is minus. Subtract 4\sqrt{65} from 28.
y=\frac{7-\sqrt{65}}{2}
Divide 28-4\sqrt{65} by 8.
y=\frac{\sqrt{65}+7}{2} y=\frac{7-\sqrt{65}}{2}
The equation is now solved.
4y^{2}-28y=16
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.
\frac{4y^{2}-28y}{4}=\frac{16}{4}
Divide both sides by 4.
y^{2}+\left(-\frac{28}{4}\right)y=\frac{16}{4}
Dividing by 4 undoes the multiplication by 4.
y^{2}-7y=\frac{16}{4}
Divide -28 by 4.
y^{2}-7y=4
Divide 16 by 4.
y^{2}-7y+\left(-\frac{7}{2}\right)^{2}=4+\left(-\frac{7}{2}\right)^{2}
Divide -7, the coefficient of the x term, by 2 to get -\frac{7}{2}. Then add the square of -\frac{7}{2} to both sides of the equation. This step makes the left hand side of the equation a perfect square.
y^{2}-7y+\frac{49}{4}=4+\frac{49}{4}
Square -\frac{7}{2} by squaring both the numerator and the denominator of the fraction.
y^{2}-7y+\frac{49}{4}=\frac{65}{4}
Add 4 to \frac{49}{4}.
\left(y-\frac{7}{2}\right)^{2}=\frac{65}{4}
Factor y^{2}-7y+\frac{49}{4}. 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(y-\frac{7}{2}\right)^{2}}=\sqrt{\frac{65}{4}}
Take the square root of both sides of the equation.
y-\frac{7}{2}=\frac{\sqrt{65}}{2} y-\frac{7}{2}=-\frac{\sqrt{65}}{2}
Simplify.
y=\frac{\sqrt{65}+7}{2} y=\frac{7-\sqrt{65}}{2}
Add \frac{7}{2} to both sides of the equation.