Question

A dog jumps straight up. Y=-16t^2+20t models the motion of the dog, where "t" is the time in seconds and "y" is the height of the dog, in feet. Use the discriminant to explain why the dolphin doesn't reach a height of 7 ft

Answer 1

Explanation:

The motion of the dolphin is quadratic

i.e y=-16t²+20t

To get the maximum height the dolphin will reach

We need to find the point of inflexion. i.e dy/dt=0

dy/dt=-32t+20

Then set dy/dt=0

0=-32t+20

Then, -32t=-20

t=0.625

Let find d²y/dt²

d²y/dt²=-32. Since this is negative then the point t=0.625 is the maximum point the dolphin can reach

Then substitute t=0.625 into y

y=-16t²+20t

y=-16(0.625)²+20(0.625)

y=6.25ft

Then the maximum height the dolphin can reach is 6.25ft

Using discriminant

Formular method

y=-16t²+20t

So the dog height y<7

y=-16t²+20t <7

-16t²+20t-7<0

a=-16, b= 20. c=-7

t=(-b±√b²-4ac)/2a

Using the a, b and c direct for the discriminant

D=b²-4ac

D=20²-4×-16×-7

D=-48

Which is a complex number

Then the dolphin can reach the height

Then we need to model the D to be greater than 0

Therefore,

D>0

b²-4ac>0

We cannot do anything to a and b it is already given

a=-16, b=20

(20)²-4(-16c)>0

400+64c>0

64c>-400

Then

c>-6.25

Divide both side by - and the inequality sign will change

Therefore -c<6.25

So the dog height y<6.25

y=-16t²+20t <6.25

Therefore the maximum height is 6.25.

If it is greater than that then, we are going to have a complex root movement which is not possible for the dolphin .