Two Hot-Air Balloons
Date: 10/21/96 at 18:4:26 From: www. Subject: Solving Two Equations Two hot-air balloons are flying over a field. The height h, in feet, of the first balloon at time t seconds is h(t)=50+34t. The height g, in feet, of the second balloon at time t is g(t)=400-16t. a. In how many seconds will the balloons be at the same height? b. How high will they be at that time? The back of my book says that the answers are (a) 7 seconds and (b) 288ft, but I can't do this myself: h(t) = 50+34t 34t = -h(t)-50 all divided by 34 t = -ht/34-1.47 h(t) = 50+34t all divided by t h = 50/t+34 g(t) = 400-16t all divided by t g = 400/t-16 g(t) = 400-16t 16t = 400-g(t) all divided by 16 t = 25-gt/16 What do I do next?
Date: 10/22/96 at 17:46:16 From: Doctor Allen Subject: Re: Solving Two Equations First - the calculations you have already performed. h(t) = 50 + 34t solving for 34t gives 34t = h(t)-50 [you had a -h(t)] h(t) and g(t) are both written to express that h and g can be calculated from a value of t. The expression, h(t), is read as "h as a function of t". Values of t can be used to find h, for example h when t=10 which is written as h(10)= 50 + 34 * 10 = 390. When you solved some of your equations, you read h(t) as h times t and cancelled the t from the expression. h(t) = 50 + 34t all divided by t h(t)/t = 50/t + 34 [you had h(t)/t = h which is not correct] The same is true for your work with the g(t) equation. Second - The problem you describe involves finding the solutions to h, g, and t for a given point (when the balloons are at the same height). To find the answer, you most combine the two equations. When the balloons are at the same height, h(t) = g(t) since h and g are the heights of the two balloons. Starting from this point, we substitute for h(t) and g(t) and solve. h(t) = g(t) 50 + 34t = 400 - 16t (substituting for h(t) and g(t)) 34t = 350 - 16 t (subtract 50 from both sides) 50t = 350 (add 16t to both sides) t = 7 (divide both sides by 50) a) The balloons will reach the same height in 7 seconds to find h(t) and g(t), substitute 7 for t in both equations h(t) = 50 + 34t h(7) = 50 + 34*7 = 288 Since the balloons are supposed to be at the same height, g should be 288 as well. Let's check. g(t) = 400 - 16t g(7) = 400 - 16*7 = 288 b) The balloons will be 288 feet up. -Doctor Allen, The Math Forum Check out our web site! http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994-2015 The Math Forum