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Question Complete the square to re-write the quadratic function in vertex form: \[ y=x^{2}-x-1 \] Answer Attempt 1 out of 2

Ask by Beck Williams. in the United States
Mar 10,2025

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Answer

The quadratic function \( y = x^2 - x - 1 \) in vertex form is: \[ y = \left(x - \frac{1}{2}\right)^2 - \frac{5}{4} \] The vertex of the parabola is at \(\left(\frac{1}{2}, -\frac{5}{4}\right)\).

Solution

To rewrite the quadratic function \( y = x^2 - x - 1 \) in vertex form, we will complete the square. Here are the steps: 1. **Identify the quadratic and linear coefficients**: The quadratic term is \( x^2 \) and the linear term is \( -x \). 2. **Factor out the coefficient of \( x^2 \)**: In this case, the coefficient is 1, so we can proceed without factoring anything out. 3. **Take half of the linear coefficient**: The linear coefficient is \(-1\). Half of \(-1\) is \(-\frac{1}{2}\). 4. **Square this value**: Squaring \(-\frac{1}{2}\) gives us \(\left(-\frac{1}{2}\right)^2 = \frac{1}{4}\). 5. **Add and subtract this square inside the equation**: We will add and subtract \(\frac{1}{4}\) within the equation to maintain equality. \[ y = x^2 - x + \frac{1}{4} - \frac{1}{4} - 1 \] 6. **Rewrite the equation**: Now we can group the perfect square trinomial and simplify the constants. \[ y = \left(x - \frac{1}{2}\right)^2 - \frac{1}{4} - 1 \] \[ y = \left(x - \frac{1}{2}\right)^2 - \frac{5}{4} \] Now, the quadratic function in vertex form is: \[ y = \left(x - \frac{1}{2}\right)^2 - \frac{5}{4} \] The vertex of the parabola represented by this function is at the point \(\left(\frac{1}{2}, -\frac{5}{4}\right)\).

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The Deep Dive

To complete the square for the quadratic function \(y = x^2 - x - 1\), we first focus on the \(x^2 - x\) part. Take the coefficient of \(x\) (which is \(-1\)), halve it to get \(-\frac{1}{2}\), then square it to get \(\frac{1}{4}\). We can rewrite the function as: \[ y = (x^2 - x + \frac{1}{4}) - \frac{1}{4} - 1 \] This simplifies to: \[ y = (x - \frac{1}{2})^2 - \frac{5}{4} \] So, the vertex form is \(y = (x - \frac{1}{2})^2 - \frac{5}{4}\). Completing the square not only helps in rewriting quadratic functions but also uncovers the vertex directly. In this case, the vertex of the parabolic graph is at \((\frac{1}{2}, -\frac{5}{4})\). This method is essential for graphing as it indicates the highest or lowest point of the curve, enhancing your understanding of its behavior. To ensure you complete the square correctly, always watch for signs! A common mistake is miscalculating the constant added or subtracted on both sides. Remember, if you add \(\frac{1}{4}\) inside the square to maintain balance, you must also subtract it out of the equation. Keeping track of your constants can save you from unnecessary algebraic headaches!

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