Big Ideas Math Algebra 2, 2014
BI
Big Ideas Math Algebra 2, 2014 View details
6. Quadratic Inequalities
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Exercise 45 Page 145

Write and solve an inequality that describes that the height of the arch when it is greater than

About m from the left pylon to about m from the left pylon.

Practice makes perfect
Let's consider the given function.
The variable describes the in meters We want to find the values of for which the arch of the bridge is above the road. From the picture, we can see that the bridge is meters above the water. Therefore, we want to find values of such that . Let's find them!
We got an quadratic inequality. There are many ways to solve it, but we will solve it algebraically. To do this, we will follow three steps.
  1. Solve the related quadratic equation.
  2. Plot the solutions on a number line.
  3. Test a value from each interval to see if it satisfies the original inequality.

Step

We will start by solving the related equation.
We see above that and Let's substitute these values into the Quadratic Formula to solve the equation.
Now we can calculate the first root using the positive sign and the second root using the negative sign.

Step

The solutions of the related equation are approximately and Let's plot them on a number line. Since the original one is a strict inequality, the points will be open.

Step

Finally, we must test a value from each interval to see if it satisfies the original inequality. Let's choose a value from the first interval, For simplicity, we will choose
Since produced a false statement, the interval is not part of the solution. Similarly, we can test the other two intervals.
Interval Test Value Statement Is it part of the solution?
Yes
No
We can now write the solution set.
Since the variable represents the distance in meters from the left pylon, the arch is above the road about m from the left pylon to about m from the left pylon.