Question 343420
Airplane Landing Speed 
To determine the appropriate landing speed of a small airplane, the formula
D= .1s^2 - 3s + 22 
is used, where s is the intitial landing speed in feet per second and D is the length of the runway in feet. If the landing speed is too fast, the pilot may run out of runway; if the speed is too slow, the plane may stall. If the runway is 800 ft long, what is the appropriate landing speed? Round to the nearest tenth.
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Solve::: 
0.1s^2 - 3s + 22 = 800
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Rearrange:
0.1s^2 - 3s - 778 = 0
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Quadratic Formula:
s = [3 +- sqrt(9 - 4*0.1*-778)]/(0.2)
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s = [3 +- sqrt(9 + 320.2)]/0.2
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Positive Solution:
s = [21.14]/0.2
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s = 105.71 mph
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Note: My calculator says the answer is 104.47 mph
Check it out.
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Cheers,
Stan H.