# Thread: help!! Function and Graphs

1. ## help!! Function and Graphs

i have problem in understanding this question. pls help me. thanks alot

solve the indicated equations graphically. assume all data are accurate to two significant digits unless greater accuracy is given.

The height h (in m) of a rocket as a function of time t (in s) of flight is given by h = 15 + 86t - 4.9t^2. Determine when the rocket is at ground level

2. Originally Posted by danielwu
i have problem in understanding this question. pls help me. thanks alot

solve the indicated equations graphically. assume all data are accurate to two significant digits unless greater accuracy is given.

The height h (in m) of a rocket as a function of time t (in s) of flight is given by h = 15 + 86t - 4.9t^2. Determine when the rocket is at ground level
the height is ground level when h=0 therefore you solve for t in the equation:

$\displaystyle -4.9t^2+86t+15=0$

now use the quadratic formula: $\displaystyle t=\frac{-b\pm\sqrt{b^2-4ac}}{2a}=\frac{-(86)\pm\sqrt{(86)^2-4(-4.9)(15)}}{2(-4.9)}$

can you solve from there?

3. is the answer = -0.2 and 17 ?

anyway don't i have to graph it out? its says solve the equations graphically.
i have tried solving but really cant graph it out. can help me thanks

4. Originally Posted by danielwu
is the answer = -0.2 and 17 ?

anyway don't i have to graph it out? its says solve the equations graphically.
i have tried solving but really cant graph it out. can help me thanks
It cannot be negative, surly?

5. sorry i don't understand what u mean. no negative?

how about the graph? how do i do it? can help me undertand futher? thanks

6. t here is time, so you cannot have the rocket reaching the ground in negative time (unless it is faster than light ).

as for the graph, try some math software. I tried some Maple (which sucks) and came up with this.

7. Originally Posted by danielwu
i have problem in understanding this question. pls help me. thanks alot

solve the indicated equations graphically. assume all data are accurate to two significant digits unless greater accuracy is given.

The height h (in m) of a rocket as a function of time t (in s) of flight is given by h = 15 + 86t - 4.9t^2. Determine when the rocket is at ground level
First have you plotted a graph of the function? It should look like the first
attachment below. From this you will observe zeros at two points one just
less than zero and the other just less than 18.

Now do new plots about 0, and 18 (the two other attached plots) and read
off the values of t which correspond to h=0 to the required precisions.

RonL