Which quadratic equation has solutions $x = 2 + \frac{1}{\sqrt{2}}$ and $x = 2 - \frac{1}{\sqrt{2}}$?
Understand the Problem
The question asks us to find the quadratic equation that has the given solutions $x = 2 + \frac{1}{\sqrt{2}}$ and $x = 2 - \frac{1}{\sqrt{2}}$. We can work backwards from the solutions to construct the quadratic equation and then identify the matching option. Alternatively, we could solve each quadratic by completing the square, but that it more complex.
Answer
$2x^2 - 8x + 7 = 0$
Answer for screen readers
$2x^2 - 8x + 7 = 0$
Steps to Solve
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Write the solutions We are given the solutions $x = 2 + \frac{1}{\sqrt{2}}$ and $x = 2 - \frac{1}{\sqrt{2}}$.
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Rewrite the solutions to obtain zero on one side Subtract the terms on the right side to get zero on one side. $x - (2 + \frac{1}{\sqrt{2}}) = 0$ and $x - (2 - \frac{1}{\sqrt{2}}) = 0$.
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Multiply the two factors Multiply the two factors together to create a quadratic equation that has the given roots. $(x - (2 + \frac{1}{\sqrt{2}}))(x - (2 - \frac{1}{\sqrt{2}})) = 0$ Expand the expression: $x^2 - x(2 - \frac{1}{\sqrt{2}}) - x(2 + \frac{1}{\sqrt{2}}) + (2 + \frac{1}{\sqrt{2}})(2 - \frac{1}{\sqrt{2}}) = 0$
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Simplify the equation Combine like terms: $x^2 - 2x + \frac{x}{\sqrt{2}} - 2x - \frac{x}{\sqrt{2}} + (4 - \frac{2}{\sqrt{2}} + \frac{2}{\sqrt{2}} - \frac{1}{2}) = 0$ $x^2 - 4x + 4 - \frac{1}{2} = 0$ $x^2 - 4x + \frac{8}{2} - \frac{1}{2} = 0$ $x^2 - 4x + \frac{7}{2} = 0$
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Multiply to remove the fraction Multiply the entire equation by $2$ to eliminate the fraction: $2(x^2 - 4x + \frac{7}{2}) = 2(0)$ $2x^2 - 8x + 7 = 0$
$2x^2 - 8x + 7 = 0$
More Information
The quadratic equation $2x^2 - 8x + 7 = 0$ has solutions $x = 2 + \frac{1}{\sqrt{2}}$ and $x = 2 - \frac{1}{\sqrt{2}}$. This can be verified by using the quadratic formula.
Tips
A common mistake is not distributing the negative sign correctly when multiplying the factors. Another common mistake is making an error when combining like terms. A final common mistake is not eliminating the denominator.
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