Math Problem Statement

A ball is launched at 42 feet per second. The height of the ball (in feet) is given by the equation h=-16t^2+48t+6, where t is the time (in seconds) since the ball was launched. For how many seconds was the ball at or above a height of 38 feet?

Solution

To find the time the ball is at or above a height of 38 feet, we start with the equation for the height of the ball:

h=16t2+48t+6h = -16t^2 + 48t + 6

We need to set this equation greater than or equal to 38:

16t2+48t+638-16t^2 + 48t + 6 \geq 38

Rearranging gives:

16t2+48t+6380-16t^2 + 48t + 6 - 38 \geq 0

16t2+48t320-16t^2 + 48t - 32 \geq 0

Dividing the entire inequality by -16 (remembering to flip the inequality sign):

t23t+20t^2 - 3t + 2 \leq 0

Next, we factor the quadratic:

(t1)(t2)0(t - 1)(t - 2) \leq 0

Now, we determine the intervals where this inequality holds. The roots are t=1t = 1 and t=2t = 2. We test the intervals around these points:

  1. Interval (,1)(-\infty, 1): Choose t=0t = 0: (01)(02)=2>0(0 - 1)(0 - 2) = 2 > 0

  2. Interval (1,2)(1, 2): Choose t=1.5t = 1.5: (1.51)(1.52)=(0.25)<0(1.5 - 1)(1.5 - 2) = (-0.25) < 0

  3. Interval (2,)(2, \infty): Choose t=3t = 3: (31)(32)=2>0(3 - 1)(3 - 2) = 2 > 0

The inequality (t1)(t2)0(t - 1)(t - 2) \leq 0 holds for:

1t21 \leq t \leq 2

Thus, the ball is at or above a height of 38 feet from t=1t = 1 second to t=2t = 2 seconds.

Duration

The duration for which the ball is at or above 38 feet is:

21=1 second2 - 1 = 1 \text{ second}

Summary

The ball is at or above a height of 38 feet for 1 second.

Would you like further details or explanations? Here are some related questions:

  1. What are the maximum height and time at which it occurs?
  2. How does the ball's height change after 2 seconds?
  3. What would happen if the launch speed were increased?
  4. How would you graph the height function?
  5. What is the total time the ball is in the air?

Tip: Always check the endpoints of your intervals when solving inequalities.

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Math Problem Analysis

Mathematical Concepts

Algebra
Quadratic Functions
Inequalities

Formulas

h = -16t^2 + 48t + 6
Quadratic inequality

Theorems

Factoring quadratics
Inequality testing

Suitable Grade Level

Grades 8-10