# help!! Function and Graphs

• August 26th 2006, 03:12 PM
danielwu
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
• August 26th 2006, 03:34 PM
Quick
Quote:

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:

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

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

can you solve from there?
• August 26th 2006, 04:29 PM
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
• August 26th 2006, 05:38 PM
ThePerfectHacker
Quote:

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?
• August 26th 2006, 09:54 PM
danielwu
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
• August 26th 2006, 11:49 PM
Rebesques
t here is time, so you cannot have the rocket reaching the ground in negative time (unless it is faster than light :confused: ).

as for the graph, try some math software. I tried some Maple (which sucks) and came up with this.
• August 26th 2006, 11:57 PM
CaptainBlack
Quote:

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