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# C1 maths help watch

1. Hey guys! So I'm working with this question

You are given that f(x) =4x
3 + kx + 6. When f(x) is divided by (x-2) the remainder is 42. Find k and hence find a root of f(x) = 0. [4]

So I've subbed two into the equation and I've found that K = 2

But I'm not sure how to go about doing the next part.

...
Trial and error.
3. As you know the remainder is 42 you subtract it from both sides of the equation to make it equal to 0 and then factorise

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4. (Original post by Joanne__)
As you know the remainder is 42 you subtract it from both sides of the equation to make it equal to 0 and then factorise

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I don't even know what you are trying to do. .
That doesn't get us any closer to finding the roots of f(x) = 0. Just use trial and error, it's likely to be in the first 5 values you try.
5. (Original post by morgan8002)
I don't even know what you are trying to do. .
That doesn't get us any closer to finding the roots of f(x) = 0. Just use trial and error, it's likely to be in the first 5 values you try.
Sub in K then set f(x) = 0

therefore: 0 = 4x^3 + 2x + 6

Now factorise

(I gtg now so I won't be able to reply, someone else can carry on from here)
6. (Original post by Dancatpro)
Sub in K then set f(x) = 0

therefore: 0 = 4x^3 + 2x + 6

Now factorise

(I gtg now so I won't be able to reply, someone else can carry on from here)
That's obvious except you can't factorise without finding a root. The OP needs to find a root. You can't factorise a cubic without first finding a root. In general find a cubic root using trial and error.

The question doesn't ask to factorise, so just find the root.
7. (Original post by morgan8002)
That's obvious except you can't factorise without finding a root. The OP needs to find a root. You can't factorise a cubic without first finding a root. In general find a cubic root using trial and error.

The question doesn't ask to factorise, so just find the root.
Heh, I don't remember trial and error in C1

Slightly worrying as a c3/c4 student
8. (Original post by Dancatpro)
Heh, I don't remember trial and error in C1

Slightly worrying as a c3/c4 student
You need it in C4 as well. When you factorise a cubic algebraically, you need to find a root via trial and error before you begin.

Most of the time they will give you a root in the first part and ask you to prove it via factor theorem, then factorise algebaically. It doesn't hurt to know how to do it from scratch though just in case.

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Updated: January 26, 2015
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