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April 8th, 2014, 01:52 AM  #1 
Member Joined: Sep 2013 Posts: 75 Thanks: 0  finding values of cubic function
f(x) =ax^3 + cx  2. The graph cuts xaxis @ (1;0) and A(2;0). Show by calculation tha a = 1 and c = 3. This is how I tried the problem. f() = 0. And f(2) =0. a(1)^3 + c(1) 2 = 0. a  c = 2. ...........(1) a(2)^3 + c(2)  2 = 0. 8a +2c = 2 ........(2). but my answer book says equation (1) is a+c = 3. and equation (2) is 8a 2c =2. where did I lost it?

April 8th, 2014, 02:00 AM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 522 Math Focus: Calculus/ODEs 
Looks to me like you did well, and the equations given by your book are wrong.

April 8th, 2014, 07:30 AM  #3 
Member Joined: Sep 2013 Posts: 75 Thanks: 0 
So I have. a  c = 2 ......(1). 8a + 2c = 2.......(2). Now multiply (1) by 2 and add the two. 2a 2c = 4. Adding. 8a +(2a) +2c +(2c) =2+4. 6a= 6. a=1. 8(1) + 2c = 2. 6 = 2c. c =3

April 8th, 2014, 07:31 AM  #4 
Member Joined: Sep 2013 Posts: 75 Thanks: 0 
I gt it


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