Question

Superman needs to save Lois from the clutches of Lex Luther. After flying for 15 seconds, he is 2100 meters from her. Then at 18 seconds he is 1980 meters from her.
A) Write an equation in slope-intercept form to model this situation where m is the distance in meters and s is the time in seconds.

B) How far is Superman away from Lois after flying for 6 seconds?

Answer 1

Part A

x = time in seconds that have gone by

y = distance in meters Superman is away from Lois

"After flying for 15 seconds, he is 2100 meters from her" means we have the point (x,y) = (15, 2100)

"at 18 seconds he is 1980 meters from her" tells us we have a second point (x,y) = (18,1980)

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Find the slope of the line through these two points

m = (y2-y1)/(x2-x1)

m = (1980-2100)/(18-15)

m = (-120)/(3)

m = -40

The slope is -40 which means Superman is getting 40 meters closer each second. I.e, the distance is dropping 40 meters per second

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Use one of the points and the slope to find the y intercept b

y = mx+b

2100 = (-40)*15 + b

2100 = -600 + b

2100+600 = b

b = 2700

This is the starting distance Superman is away from Lois

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Since m = -40 and b = 2700, we know the y = mx+b equation becomes y = -40x+2700.

Replace x with s, replace y with m. We now have m = -40s+2700

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Final Answer:  m = -40s+2700

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Part B

Plug s = 6 into the equation we found for part A. Then simplify.

m = -40s+2700

m = -40*6+2700

m = -240+2700

m = 2460

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Final Answer: 2460 meters