The police department uses a formula to determine the speed at which a car was going when the driver applied the
brakes, by measuring the distance of the skid marks. The
equation d = 0.0015r^2 + r models the distance, d, in feet
traveled after applying the brakes in a car moving
r miles per hour (r is the speed of the car).
If the distance the car traveled after applying the brakes
equals 88 feet, what, what was the speed of the car? *
*Round your answer to the nearest hundredth miles per hour.

mph

9514 1404 393

  78.71 mph

Step-by-step explanation:

You're looking for the solution to ...

  d = 88

  0.0015r^2 +r = 88

  0.0015r^2 +r -88 = 0

Using the quadratic formula*, we find ...

  r = (-1 +√(1² -4(0.0015)(-88)))/(2×0.0015)

  r = (-1 +√1.528)/0.003 = 0.236123/0.003 = 78.7077

The speed of the car was about 78.71 mph.

_____

* The quadratic formula will also give a negative answer of about -745 mph, which is not relevant to the situation.

step-by-step explanation:

this can be solved first with pythagorean theorem, which tells us to square both legs of the triangle and sum it. this will be equal to the square of the hypotenuse.

the hypotenuse is the side "opposite" the 90 degree angle, which is x.

so we can write using pythagorean theorem,

simplifying:

to further simplify this radical, we need to use the property  

thus