# C12 Binomial ExpansionWatch

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#1
f(x)=(a-x)(3+ax)^5

The first question is to find the first 3 terms and give them in the simplest form.

The second question is to find the exact value of a, given that the co-efficient of x is zero in the expansion of f(x)

Can someone pls help me with the second question
0
3 weeks ago
#2
What did you get for the first part, as the 2nd should be simply setting the x coefficient to zero?
(Original post by Ellie.Ginger)
f(x)=(a-x)(3+ax)^5

The first question is to find the first 3 terms and give them in the simplest form.

The second question is to find the exact value of a, given that the co-efficient of x is zero in the expansion of f(x)

Can someone pls help me with the second question
0
#3
(Original post by mqb2766)
What did you get for the first part, as the 2nd should be simply setting the x coefficient to zero?
I got 243+405ax+270a^2 x^2
0
3 weeks ago
#4
(a-x)(3+ax)^5
Did you multiply it by the first expression in brackets?
(Original post by Ellie.Ginger)
I got 243+405ax+270a^2 x^2
0
#5
(Original post by mqb2766)
(a-x)(3+ax)^5
Did you multiply it by the first expression in brackets?
oh I'm so sorry I didn't mention. We were asked to find the first 3 terms of (3+ax)^5 only. then in the second question only we had to use the entire f(x)
0
3 weeks ago
#6
So what did you get for the linear term in the 2nd part?
(Original post by Ellie.Ginger)
oh I'm so sorry I didn't mention. We were asked to find the first 3 terms of (3+ax)^5 only. then in the second question only we had to use the entire f(x)
0
#7
(Original post by Ellie.Ginger)
I got 243+405ax+270a^2 x^2
This
0
3 weeks ago
#8
Eh?
The expansion of (3+ax)^5 is
243+405ax+270a^2 x^2
Multply that by (a-x)
What is the linear "x" term?

(Original post by Ellie.Ginger)
This
0
#9
(Original post by mqb2766)
Eh?
The expansion of (3+ax)^5 is
243+405ax+270a^2 x^2
Multply that by (a-x)
What is the linear "x" term?
I don't know how to do the second part of the question... so idk
0
3 weeks ago
#10
Expand
(a-x)*(243+405ax+270a^2 x^2)
and collect terms (constant, linear, quadratic, ...).

(Original post by Ellie.Ginger)
I don't know how to do the second part of the question... so idk
0
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