C2: Chapter 5

Binomial Expansion

Dr J Frost ([email protected]) Last modified: 22 nd September 2013

Starter a) Expand (a + b) 0 b) Expand (a + b) 1 c) Expand (a + b) 2 d) Expand (a + b) 3 e) Expand (a + b) 4 1a 4 1a 3 1a 2 + 3a + 4a 3 2 1 ?

?

?

?

?

2 b 2 2 2 + 1b 3 + 4ab 3 + b 4 What do you notice about:

The coefficients: The powers of a and b:

They follow Pascal’s triangle.

?

Power of a decreases each time (starting at the power) ?

Quickfire Pascal What coefficients in your expansion do you use if the power is: 2:

1 2 1

?

Quickfire Pascal What coefficients in your expansion do you use if the power is: 4:

Quickfire Pascal What coefficients in your expansion do you use if the power is: 3:

Quickfire Pascal What coefficients in your expansion do you use if the power is: 5:

Quickfire Pascal What coefficients in your expansion do you use if the power is: 2:

1 2 1

?

Quickfire Pascal What coefficients in your expansion do you use if the power is: 4:

Quickfire Pascal What coefficients in your expansion do you use if the power is: 3:

Quickfire Pascal What coefficients in your expansion do you use if the power is: 5:

Quickfire Pascal What coefficients in your expansion do you use if the power is: 4:

Binomial Expansion (x + 2y) 4 = x 4 + x 3 + x 2 2 + x + (2y) 4 = x 4 + 8x 3 y + 24x 2 y 2 + 32xy 3 + 16y 4 Step 1: You could first put in the first term with decreasing powers.

Step 2: Put in your second term with increasing powers, starting from 0 (i.e. so that 2y doesn’t appear in the first term of the expansion, because the power is 0) Step 3: Add the coefficients according to Pascal’s Triangle.

Your go...

(2x – 5) 3 = (2x) 3 = 8x 3 + 3(2x) 2 (-5) + 3(2x)(-5) 2 – 60x 2 + 150x – 125 + (-5) 3 Bro Tip: If one of the terms in the bracket is negative, the terms in the result will oscillate between positive and negative.

The coefficient of x 2 in the expansion of (2 – cx) 3 Find the possible value(s) of the constant c.

(2 – cx) 3 = 2 3 + 3 2 2 (-cx) + 3 2 1 (-cx) 2 So coefficient of x 2 c =



7 is 6c 2

?

= 294 + ...

is 294. Bro Tip: When asked about a particular term, it’s helpful to write out the first few terms of the expansion, until you write up to the one needed. There’s no point of simplifying the whole expansion!

Exercises

Page 79 Exercise 5A

1c, d, g, h 2d, g 3, 4, 6

How are the rows of Pascal’s Triangle generated?

How many ways are there of choosing 0 items from 4?

= 4 C 0 =

𝟒 𝟎 ?

= 𝟏 How many ways are there of choosing 1 item from 4?

= 4 C 1 =

𝟒 𝟏 ?

= 𝟒 How many ways are there of choosing 2 items from 4?

= 4 C 2 =

𝟒 𝟐 ?

= 𝟔 How many ways are there of choosing 3 items from 4?

= 4 C 3 =

𝟒 𝟑 ?

= 𝟒 How many ways are there of choosing 4 items from 4?

= 4 C 4 =

𝟒 𝟒 ?

= 𝟏

1 1 1 1 2 1 1 3 3 1

1 4 6 4 1

1 5 10 10 5 1

Binomial Coefficients This is known as a binomial coefficient. It can also be written as n C r (said: “n choose r”) ?

?

?

?

?

?

?

?

Binomial Coefficients To calculate Binomial Coefficients easily: ?

?

?

Because when we divide 8! by 6!, we cancel out all the numbers between 1 and 6 in the product.

i.e. The bottom number of the binomial coefficient (2) tells us how many consecutive numbers we multiply together.

?

?

General formula Edexcel May 2013 (Retracted) ?

Using Binomial Expansions for approximations Edexcel Jan 2012 ?

If , then x = 0.1. Plugging this in to our expansion: 1 + 0.2 + 0.0175 + 0.00875 = 1.218375

Actual value is (1.025) 8 = 1.218403. So it is correct to 4dp!

Using Binomial Expansions for approximations Exercise 5C Q7 Write down the first four terms in the expansion of By substituting an appropriate value for x, find an approximate value to (0.99) 6 . Use your calculator to determine the degree of accuracy of your approximation.

1 – 0.6x + 0.15x

2 – 0.02x

3 0.94148, which is accurate to 5dp

?

Q8 Write down the first four terms in the expansion of By substituting an appropriate value of x, find an approximate value to (2.1) 10 . Use your calculator to determine your approximation’s degree of accuracy.

1024 + 1024x + 460.8x

2 + 122.88x

3 1666.56, which is accurate to 3sf

?

Using Binomial Expansions for approximations Edexcel January 2007 a) 1 + 5(-2x) + 10(-2x) 2 = 1 – 10x + 40x 2 + 10(-2x) – 80x 3 ?

3 b) We discard the x  2 and x 3 terms above.

?

2 = 1 – 9x – 10x 1- 9x (since we can discard the x 2 2 term again)