Math Is Fun Forum

Spanky

Member

Registered: 2010-02-24

Posts: 62

Quadratic Formual

Use the Quadratic Formula to solve:

18r^2 -450r -994375=0

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Quadratic Formual

High Spanky;

For the general quadratic:

Your quadratic:

So
a  = 18
b  = -450
c  = -994375

Now just plug into A and B. Tell me where you get stuck.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Spanky

Member

Registered: 2010-02-24

Posts: 62

Re: Quadratic Formual

Thanks bobby, but that's as far as i got.  I cant seem to get the answer.  I'm coming up with

x= 450 +- square root of 71797500 / 36

(Sorry, I dont know how to use the square root symbol on here)

soroban

Member

Registered: 2007-03-09

Posts: 452

Re: Quadratic Formual

.

. .

.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Quadratic Formual

Hi Spanky;

I don't think this is any simpler than soroban's answer just providing it for when you check the answer.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Spanky

Member

Registered: 2010-02-24

Posts: 62

Re: Quadratic Formual

Hi Spanky;

Ok, maybe i should give you the full problem because the answer is supposed to be 247.87

Apply the Pythagorean theorem: Two planes depart simultaneously from an airport.  One flies due North, the other due west at a rate of 25 mi/h slower than the first plane. After 3 hours, radar indicates that the planes are 1000 mi apart.  What is the ground speed of each plane?  Let r be the ground speed of the first plane.

So I did:
Let r= the ground speed of first plane
r - 25 = the speed of second plane

a^2 + b^2 = c^2

(3r)^2 + [ 3(r-25)]^2 = (1000)^2

9r^2 + (3r-75)^2 = 1000000

9r^2 + (9r^2 - 450r + 5625) = 1000000

18r^2- 450r -994375= 0

Then I was going to use the Quadratic Formula to solve for the answer, which should turn out to be 247.87 mi/h.  Where did I make my mistake?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Quadratic Formual

You ignore the negative answer because you cannot have a negative velocity.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Spanky

Member

Registered: 2010-02-24

Posts: 62

Re: Quadratic Formual

Oh, Ok I see now.  Can you explain to me how you turned 75 + 25 square root 3191 / 6 into 25(3 + square root 3191) / 6?  I'm not seeing it...

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Quadratic Formual

Hi Spanky;

By the distributive property.

ab + ac = a(b + c)

(75 + 25 √3191) = (3*25 + 25 √3191) Now you can pull out a 25 from both terms.

25(3 + √3191)

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Spanky

Member

Registered: 2010-02-24

Posts: 62

Re: Quadratic Formual

Thanks!