SOLUTION: Lisa traveled 1040 mi at a certain speed. If the speed had been 13 mph faster, the trip could have been in 4 hr. less time. Find the speed.

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Question 909916: Lisa traveled 1040 mi at a certain speed. If the speed had been 13 mph faster, the trip could have been in 4 hr. less time. Find the speed.
Answer by ewatrrr(24785)   (Show Source): You can put this solution on YOUR website!
Set Up
|Multiplying thru by r(r+13) so as all denominators = 1
1040(r+13) - 1040r = 4r^2 + 52r
4r^2 +52r - 13(1040) = 0
r^2 + 13r - 13(260) = 0
r^2 + 13r - 3380 = 0 (Tossing out the negative solution for unit measure)
r = 52mph. 65mph to save 4hrs
Solved by pluggable solver: SOLVE quadratic equation with variable
Quadratic equation (in our case ) has the following solutons:



For these solutions to exist, the discriminant should not be a negative number.

First, we need to compute the discriminant : .

Discriminant d=13689 is greater than zero. That means that there are two solutions: .




Quadratic expression can be factored:

Again, the answer is: 52, -65. Here's your graph:
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