Chapter 9.1, Problem 4CGP

To determine

To calculate the domain and range for the given function.

Answer to Problem 4CGP

The domain is all real numbers, that is, $\left\{-\infty <x<\infty \right\}$ and range is all values greater than or equal to -3, that is, $\left\{y|y\ge -3\right\}$ .

Explanation of Solution

Given information :

The function provided is $y=2x^2-4x-1$ .

Formula used :

Formula to compute the x-coordinate of the vertex is $x=-\frac{b}{2\times a}$ . Here, $a=2$ and $b=-4$ . ‘a’ and ‘b’ are coefficients $x^2$ and $x$ respectively. The corresponding value of ‘y’ for the vertex would be the maximum or minimum.

Calculation :

Since the coefficient of ‘a’ is positive, the curve of this function is upward facing and hence the function has a minimum value.

X-coordinate of the vertex is

  $x=-\frac{b}{2\times a}$

Formula for axis of symmetry.

  $x=-\frac{-4}{2\times \left(2\right)}$

Putting the values of ‘a’ and ‘b’ .

  $=\frac{4}{4}$

Simplifying

  $x=1$

Vertex can be found out by putting the value of x computed in the axis of symmetry in the original function. This will give a value of y , that will be the minimum.

  $=2{\left(1\right)}^2-4\left(1\right)-1$

Putting the value of $x=1$ in the original function.

  $=2-4-1$

Simplifying the expression.

  $=-3$

Thus, the minimum is at -3 .

Since x can take up any real values, the range is $\left\{-\infty <x<\infty \right\}$ .

Since the domain is the values of y, it can only take up values greater than or equal to the minimum. That is, $\left\{y|y\ge -3\right\}$ .