Over two summers, Ray saved $900 and $500. The polynomial 900x2 + 500x represents his savings at the beginning of the third year, where x is the growth factor. (The interest rate r is x - 1.) What is the interest rate he needs to start the third year with $1600 in savings?

Question

anonymous · Answer

@rajee_sam 
@dan815 
@Numb3r1

anonymous · Answer

That means 900x^2+500x=1600, which means 900x^2+500x-1600=0. Factor and solve!

anonymous · Answer

I should look for x

rajee_sam · Answer

after you find x the interest rate should be x-1

anonymous · Answer

okay 1 sec

anonymous · Answer

I got 2 solutions

rajee_sam · Answer

one will be negative and one will be +ve

rajee_sam · Answer

I cannot have negative growth rate so eliminate that

anonymous · Answer

A. 1.1%	

B. 0.1%	

C. 8.4%	

D. 264%

anonymous · Answer

these are my solutions http://www.wolframalpha.com/input/?i=900x%5E2%2B500x-1600%3D0

anonymous · Answer

Just looking at D it is way too high. B is too low. Now, I can look at these and tell which is correct, but it's better to take A and C and see which works with the math.

anonymous · Answer

That doesn't work.

anonymous · Answer

That's x^2+7x-16, not 5x.

rajee_sam · Answer

no 900x^2 + 500x = 1600 so it will be 9x^2 + 5x -16 = 0 not 7x

rajee_sam · Answer

I did my factoring wrong my bad

anonymous · Answer

http://www.wolframalpha.com/input/?i=9x%5E2%2B5x-16%3D0 these are the right solutions

rajee_sam · Answer

1.1 is you x so r = 1.1 - 1 = .1%

rajee_sam · Answer

yes your solutions are right

rajee_sam · Answer

if you look at both your solutions first one is negative. second one is positive.

So consider that. if you calculate that it turns out to be 1.1

rajee_sam · Answer

So Answer is B

rajee_sam · Answer

if x = 1.1 then r = x-1

anonymous · Answer

yeah tha't correct I was thinking about that one thanks all

arihangdu · Answer

Is this correct?