Chapter 9: Problem 66
Write the quadratic function $$ y=8 x^{2}-2 x-15 $$ in the forms indicated.Give the values of all constants. $$ y=a(x-r)(x-s) $$
Short Answer
Expert verified
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**Answer: The values of the constants are $$a=8$$, $$r=\frac{3}{2}$$, and $$s=-\frac{5}{4}$$.**
Step by step solution
01
Find the Roots of the Quadratic Equation
To find the roots of the quadratic equation $$8x^2 - 2x - 15$$, we can use the quadratic formula: $$x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}$$, with $$a=8, b=-2, c=-15$$. Plugging these values into the formula, we get:$$
x=\frac{2 \pm \sqrt{(-2)^{2} - 4 \cdot 8 \cdot (-15)}}{2 \cdot 8}
$$
02
Calculate the Discriminant
Before solving for x, calculate the discriminant:$$
\Delta = b^2 - 4ac = (-2)^2 - 4(8)(-15) = 4 + 480 = 484
$$The discriminant is $$\Delta = 484$$, which is a positive perfect square, meaning we will have two real and rational roots.
03
Solve for x using Quadratic Formula
Now, we can solve for x with the discriminant:$$
x = \frac{2 \pm \sqrt{484}}{16}
$$Simplifying, we get:$$
x = \frac{2\pm22}{16}
$$Thus, we have two roots:$$
x_1 = \frac{(2+22)}{16}=\frac{24}{16}= \frac{3}{2}\\
x_2 = \frac{(2-22)}{16}=\frac{-20}{16}=-\frac{5}{4}
$$
04
Write the quadratic equation in the factored form
Now that we have both roots $$x_1 = \frac{3}{2}$$ and $$x_2 = -\frac{5}{4}$$, we can rewrite the quadratic equation in the factored form:$$
y = a(x-x_1)(x-x_2)\\
y = a(x-\frac{3}{2})(x+\frac{5}{4})
$$To find the value of a, we can use an original equation and plug one of the roots with its corresponding y-coordinate, for example, use the root $$x_1 = \frac{3}{2}$$ :$$
y = 8(\frac{3}{2})^2 - 2(\frac{3}{2}) - 15
$$Solving, we get:$$
y = 8 \cdot \frac{9}{4} - 3 - 15 = 18 - 3 - 15 = 0
$$Since we are trying to find the original equation, the y value will be 0, so:$$
0 = a(x_1-\frac{3}{2})(x_1+\frac{5}{4})
$$With $$x_1 = \frac{3}{2}$$, we have:$$
0 = a(\frac{3}{2} - \frac{3}{2})(\frac{3}{2} + \frac{5}{4})
$$Since the left side is 0, the right side of the equation must also be 0. Given the factor of $$x_1 - \frac{3}{2}$$ will be equal to 0, the value of "a" can be anything. We use the coefficient of $$x^2$$ term in the original equation:$$
a=8
$$
05
Finalize the factored form of the quadratic equation
So, finally, the quadratic equation in its factored form is:$$
y = 8(x-\frac{3}{2})(x+\frac{5}{4})
$$The constants in the factored form are $$a=8$$, $$r=\frac{3}{2}$$, and $$s=-\frac{5}{4}$$.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Factored Form
The factored form of a quadratic equation is a way to express it as a product of its linear factors. This representation is helpful since it directly shows the roots of the quadratic equation. If a quadratic equation is written as \(y = ax^2 + bx + c\), then its factored form will look like \(y = a(x - r)(x - s)\). Here:
- \(a\) is a constant that stretches or compresses the parabola along the y-axis.
- \(r\) and \(s\) are the roots or solutions of the quadratic equation.
Quadratic Formula
The quadratic formula is a powerful tool used to find the roots of any quadratic equation of the form \(ax^2 + bx + c = 0\). The formula is \(x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}\), and it uses three coefficients:
- \(a\) is the coefficient of \(x^2\), which determines the parabola's direction (upward if \(a > 0\), downward if \(a < 0\)).
- \(b\) is the coefficient of \(x\), which affects the position of the vertex of the parabola.
- \(c\) is the constant term that influences where the parabola intersects with the y-axis.
Discriminant
The discriminant is a component derived from the quadratic formula that reveals crucial information about the roots of a quadratic equation. It is computed as \(\Delta = b^2 - 4ac\). This value determines the nature of the roots for the given quadratic:
- If \(\Delta > 0\), the quadratic equation has two distinct real and rational roots.
- If \(\Delta = 0\), there is exactly one real root, indicating the parabola touches the x-axis at its vertex.
- If \(\Delta < 0\), the roots are complex and do not intersect the x-axis.