# Binomial expansion question. Please help:(((

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Thread starter 4 weeks ago
#1
Pls help It is a textbook question and the answer is -1 and 5/2. I am so lost. All replies are much appreciated.
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4 weeks ago
#2
(Original post by Morgana:))
Pls help It is a textbook question and the answer is -1 and 5/2. I am so lost. All replies are much appreciated.
Post your working.
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4 weeks ago
#3
(Original post by Morgana:))
Pls help It is a textbook question and the answer is -1 and 5/2. I am so lost. All replies are much appreciated.
You have a product of two terms.

The overall coefficient of x^3 will be given by:

(constant from first factor) * (x^3 term from second factor)
+
(x term from first factor) * (x^2 term from second factor)

and if the first factor had a quadratic term, we would continue this chain.

Anyway, you need to simply work out what the cubic and quadratic x terms are in terms of b, and use them in equating the overall coeff of x^3 to 45.
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Thread starter 4 weeks ago
#4
(Original post by Plücker)
Post your working.
i wish you don't mean anything meanhere you go
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Thread starter 4 weeks ago
#5
(Original post by RDKGames)
You have a product of two terms.

The overall coefficient of x^3 will be given by:

(constant from first factor) * (x^3 term from second factor)
+
(x term from first factor) * (x^2 term from second factor)

and if the first factor had a quadratic term, we would continue this chain.

Anyway, you need to simply work out what the cubic and quadratic x terms are in terms of b, and use them in equating the overall coeff of x^3 to 45.
Many thanks!!!! but I calculated three to four times and always got 2b^3 + 9b^2 = 45, but the answer is -1 and 5/2 which don't fit. I am so stuck
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4 weeks ago
#6
(Original post by Morgana:))
Pls help It is a textbook question and the answer is -1 and 5/2. I am so lost. All replies are much appreciated.
I agree with the working you posted. Question seems to have a misprint. You get the values asked for if you find the coefficient of x^2 instead of x^3.
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4 weeks ago
#7
(Original post by Morgana:))
i wish you don't mean anything meanhere you go
He's just saying that because people often post questions without working and expect others to do their work for them!
(Original post by Morgana:))
Many thanks!!!! but I calculated three to four times and always got 2b^3 + 9b^2 = 45, but the answer is -1 and 5/2 which don't fit. I am so stuck
I get 2b^3 - 9b^2 = 45 which also doesn't match those solutions. Is this the complete question or is there more to it?

Where's the question from? It's not unknown for textbooks to have errors in them!!
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4 weeks ago
#8
(Original post by Morgana:))
Pls help It is a textbook question and the answer is -1 and 5/2. I am so lost. All replies are much appreciated.
UPDATE: looks like a misprint. Try doing the question again but pretend they wrote "coefficient of x^2" not x^3
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Thread starter 4 weeks ago
#9
(Original post by davros)
UPDATE: looks like a misprint. Try doing the question again but pretend they wrote "coefficient of x^2" not x^3
Ahhh thank you so much!!!!!!!! It's from the Edexcel Pure mathematics textbook, I also doubt it is a misprint🤔and your method makes so much sense
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4 weeks ago
#10
(Original post by Morgana:))
i wish you don't mean anything meanhere you go
Not at all. I'm hardly ever mean.
I usually write

Post your working?

so that it doesn't sound like an order.
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