How long does it take the giraffe to weigh 85 kg?

1. Mar 30, 2008

Paulo2014

1. The problem statement, all variables and given/known data
The weight, W kg, of a giraffe over its first two years of life, is given by the equation
W=(t^2)/4-t+68
where t is the time in months since the giraffe was born.
How long does it take the giraffe to weigh 85 kg?

2. Relevant equations

3. The attempt at a solution

I know i need to rearage the equation but i cant remember how so any help is appreciated...

Last edited by a moderator: Jul 18, 2014
2. Mar 30, 2008

happyg1

Which letter represents the weight? Put 85 there and solve it.

3. Mar 30, 2008

Paulo2014

Yeah I understand that but how would I solve it?

its the (t^2)/4 bit i don't understand how to solve...

4. Mar 30, 2008

happyg1

Let me see if this is the thing you have:
$$\frac{t^2}{4}-t+68=85$$
That's a quadratic equation. Before you can solve, what does the equation have to look like?
Then, since fractions give me a headache, like like to ditch them. Can you think of a method of getting rid of them?

5. Mar 30, 2008

Paulo2014

multiply 4 to both sides?

6. Mar 30, 2008

happyg1

7. Mar 30, 2008

Paulo2014

So it would be:
t^2-t+68=340?

8. Mar 30, 2008

happyg1

so you have:
$$4(\frac{t^2}{4}-t+68=85)$$
EVERYTHING gets multiplied by 4.

9. Mar 30, 2008

Paulo2014

but doesn't the t^2 already have the /4 so wouldn't it be:

t^2+4(-t+68=85)?

10. Mar 30, 2008

happyg1

yes
$$4(\frac{t^2}{4}-t+68=85)$$
$$t^2-(4)t+(4)68=(4)85$$
then what?

11. Mar 30, 2008

Paulo2014

expand.

So it is t^2-4t+272=340

Is that right?

12. Mar 30, 2008

happyg1

yes.
In order to solve ANY equation, what does it have to be set equal to?
There's ONE MORE STEP before you can solve it.

13. Mar 30, 2008

Paulo2014

I can't remember... :(

14. Mar 30, 2008

rocomath

Look for an example in your book.

15. Mar 30, 2008

Paulo2014

It has to be in the form ax=b

thats what my textbook says.... is that what you meant?

16. Mar 30, 2008

happyg1

You are looking for the ZEROS
What would you set the equation equal to to get the ZEROS?

17. Mar 30, 2008

Paulo2014

i'm lost....

18. Mar 30, 2008

happyg1

In order to solve any equation, it must be set equal to ZERO!
HOORAY!
How do you manipulate your equation to get it equal to zero?

19. Mar 30, 2008

Paulo2014

I see. take 340 from both sides so the equation is:t^2-4t-68=0 is that correct?

20. Mar 30, 2008

rocomath

Sit down and read the chapter ... do every single example problem!!! It will serve you better than trying to solve this problem.