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April 8th, 2014, 01:52 AM   #1
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finding values of cubic function

f(x) =ax^3 + cx - 2. The graph cuts x-axis @ (-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?
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April 8th, 2014, 02:00 AM   #2
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Looks to me like you did well, and the equations given by your book are wrong.
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April 8th, 2014, 07:30 AM   #3
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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
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April 8th, 2014, 07:31 AM   #4
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I gt it
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