Binomial Theorem Formula – General Term
By the Binomial theorem formula, we know that there are (n + 1) terms in the expansion of (a+b)n. Now, let’s say that T1, T2, T3, T4, … Tn+1 are the first, second, third, fourth, … (n + 1)th terms, respectively in the expansion of (a+b)n. Therefore,
Generalizing it, we have the formula for the general term:
where 0 ≤ r ≤ n. Let’s look at an example now.
Example 1
Find the fourth term in the expansion of (3x–y)7.
Solution. In this example, a = 3x, b = – y, and n = 7. From the above formula, we have
To find the fourth term, T4, r = 3. Therefore,
Hence, the fourth term in the expansion of (3x–y)7 = – 2835x4y3
Binomial Theorem Formula – Middle Term
When you are trying to expand (a+b)n and ‘n’ is an even number, then (n + 1) will be an odd number. Which means that the exapnsion will have odd number of terms. In this case, the middle term will be the (n/2 + 1)th term.
For example, if you are expanding (x+y)2, then the middle term will be the (2/2 + 1) = 2nd term. (n/2 + 1)th term is also denoted as (n+2/2)th term.
When you are trying to expand (a+b)n and ‘n’ is an odd number, then (n + 1) will be an even number. Hence, there are two middle terms: (n+1/2)th term and (n+3/2)th term.
For example, if you are expanding (x+y)3, then the middle terms will be, (3+1/2) = 2nd term and (3+3/2) = 3rd term. Let’s look at an example now.
Example 2
Find the middle term/s in the expansion of (x/2 + 3/y)9.
Solution. In this example, since n (= 9) is odd, we have two middle terms namely,
(9+1/2) = 5th term and (9+3/2) = 6th term.
We also have,
a = x/2, b = 3y, and n = 9.
We know that,
To find the fifth term, T5, r = 4. Therefore,
Similarly,
Therefore, the middle terms in the expansion of (x/2 + 3y)9 are 5103/16 x5y4 and 15309/8
x4y5.
More Solved Examples
Question: Find the coefficient of x6 in the expansion of (x+2)9.
Solution: We know that the binomial expansion of (a+b)n is,
Now, the (r + 1)th term in the expansion of (x+2)9 will be:
From the above equation, we can deduce that the coefficient of the x-term is
Next, we need to find the coefficient of x6. Hence,
x6 = (x)9–r
Or, 6 = 9 – r, therefore r = 3.
Using this value of ‘r’,
The coefficient of x6 =
= 9.8.7/1.2.3.(8) = 672.
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1. What is the binomial theorem? |
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