To identify which scenario involves a quadratic equation, let's analyze each of the scenarios you've presented:
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Coach models the trajectory of a ball: This scenario deals with the motion of a ball, which is typically modeled by a quadratic equation (specifically a parabolic equation) in physics. The height of the ball as a function of time, under the influence of gravity, is often described by a quadratic function. Therefore, this scenario would use a quadratic equation.
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Savings account that doubles every month: This scenario represents exponential growth. An account that doubles its amount every period can be modeled by an exponential equation, not a quadratic equation.
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A balloon that rises 2 ft per second: This scenario describes a linear motion since the height of the balloon increases at a constant rate (2 ft per second). Thus, this can be expressed with a linear equation, not a quadratic equation.
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A child places a quarter into a piggy bank every week: This is also a linear scenario since the amount of money in the piggy bank increases at a constant rate (a quarter per week). It can be expressed with a linear equation as well.
Based on the analysis, the scenario that uses a quadratic equation is:
The coach modeling the trajectory of a ball.