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

1. Homework Statement
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. Homework 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...

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

Yeah I understand that but how would I solve it?

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

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?

multiply 4 to both sides?

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

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

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

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

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

expand.

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

Is that right?

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.

I can't remember... :(

Look for an example in your book.

It has to be in the form ax=b

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

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

i'm lost....

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?

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

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

rocomath is right.
and you did get the equation right. If you aren't sure how to solve it, you DO need to read the chapter.

I use the quadratic formula don't I...

yes you do

I got x=10.48528137423857 or -6.4852813742385695

Is that right?