The Binomial Theorem

Pascal’s Triangle and the Binomial Theorem

(x + y)0 = 1

(x + y)1 = 1x + 1y(x + y)2 = 1x2 + 2xy + 1y2

(x + y)3 = 1x3 + 3x2y + 3xy2 +1 y3

(x + y)4 = 1x4 + 4x3y + 6x2y2 + 4xy3 + 1y4

(x + y)5 = 1x5 + 5x4y + 10x3y2 + 10x2y3 + 5xy4 + 1y5

(x + y)6 = 1x6 + 6x5y1 + 15x4y2 + 20x3y3 + 15x2y4 + 6xy5 + 1y6

The Binomial Theorem is a formula used for expanding powers of binomials.

(a + b)3 = (a + b)(a + b)(a + b) = a3 + 3a2b + 3ab2 + b3

Each term of the answer is the product of three first-degree factors. For each term of theanswer, an a and/or b is taken from each first-degree factor.

• The first term has no b. It is like choosing no b from three b’s. The combination 3C0 is the coefficient of the first term

• The second term has one b. It is like choosing one b from three b’s. The combination 3C1 is the coefficient of the second term

• The third term has two b’s. It is like choosing two b’s from three b’s. The combination 3C2 is the coefficient of the first term• The fourth term has three b’s. It is like choosing three b’s from three b’s. The combination 3C3 is the coefficient of the third term

(a + b)3 = 3C0 a3 + 3C1 a2b + 3C2 ab2 + 3C3 b3

The Binomial Theorem

1st Row

2nd Row

3rd Row

4th Row

5th Row

0th Row

The numerical coefficients in a binomial expansion can be found in Pascal’s triangle.

(a + b)n

0C0

1C01C1

2C02C1

2C2

3C03C1

3C23C3

4C04C1

4C24C3

4C4

5C05C1

5C25C3

5C45C5

Pascal’s Triangle and the Binomial Theorem

1

Pascal’sTriangle

1 1

1 2 1

1 3 3 1

1 4 6 4 11 5 10 10 5 1

Pascal’s TriangleUsing Combinatorics

(a + b)3 = a3 + 3a2b + 3ab2 + b3

The degree of each term is 3.For the variable a, the degree descends from 3 to 0.For the variable b, the degree ascends from 0 to 3.

(a + b)3 = 3C0 a3 - 0b0 + 3C1 a3 - 1b1 + 3C2 a3 - 2b2 + 3C3 a3 - 3b3

(a + b)n = nC0 an - 0b0 + nC1 an - 1b1 + nC2 an - 2b2 + … + nCk an - kbk

The general term is the (k + 1)th term:

tk + 1 = nCk an - k bk

Binomial Expansion - the General Term

Expand the following.

a) (3x + 2)4 = 4C0(3x)4(2)0 + 4C1(3x)3(2)1

+ 4C2(3x)2(2)2 + 4C3(3x)1(2)3 + 4C4(3x)0(2)4

n = 4a = 3x b = 2

= 1(81x4) + 4(27x3)(2) + 6(9x2)(4)+ 4(3x)(8) + 1(16)

= 81x4 + 216x3 + 216x2 + 96x +16

Binomial Expansion - Practice

Expand the following.

b) (2x - 3y)4

= 4C0(2x)4(-3y)0

+ 4C3(2x)1(-3y)3

= 1(16x4)

= 16x4 - 96x3y + 216x2y2 - 216xy3 + 81y4

n = 4a = 2x b = -3y

Binomial Expansion - Practice

+ 4C1(2x)3(-3y)1 + 4C2(2x)2(-3y)2

+ 4C4(2x)0(-3y)4

+ 4(8x3)(-3y) + 6(4x2)(9y2) + 4(2x)(-27y3) + 81y4

Binomial Expansion - Practice

2x

3x

5

(2x - 3x-1)5

= 5C0(2x)5(-3x-1)0 + 5C1(2x)4(-3x-1)1 + 5C2(2x)3(-3x-1)2

+ 5C3(2x)2(-3x-1)3 + 5C4(2x)1(-3x-1)4 + 5C5(2x)0(-3x-1)5

n = 5a = 2x b = -3x-1

= 1(32x5) + 5(16x4)(-3x-1) + 10(8x3)(9x-2) + 10(4x2)(-27x-3)+ 5(2x)(81x-4) + 1(-81x-5)

= 32x5 - 240x3 + 720x1 - 1080x-1 + 810x-3 - 81x-5

c)

a) Find the eighth term in the expansion of (3x - 2)11.

tk + 1 = nCk an - k bk

n = 11a = 3xb = -2k = 7t7 + 1 = 11C7 (3x)11 - 7

(-2)7

t8 = 11C7 (3x)4 (-2)7

= 330(81x4)(-128) = -3 421 440 x4

b) Find the middle term of (a2 - 3b3)8.

n = 8a = a2

b = -3b3

k = 4

n = 8, therefore, there are nine terms. The fifth term is the middle term.tk + 1 = nCk an - k

bk

t4 + 1 = 8C4 (a2) 8 - 4 (-3b3)4

t5 = 8C4 (a2) 4 (-3b3)4

= 70a8(81b12) = 5670a8b12

Finding a Particular Term in a Binomial Expansion

Finding a Particular Term in a Binomial Expansion

2x 1x2

18

.Find the constant term of the expansion of

tk + 1 = nCk an - k bk

n = 18a = 2xb = -x-2

k = ?tk + 1 = 18Ck (2x)18 - k

(-x-2)k

tk + 1 = 18Ck 218 - k x18 - k (-1)k x-2k

tk + 1 = 18Ck 218 - k (-1)k x18 - k x-2k

tk + 1 = 18Ck 218 - k (-1)k x18 - 3k

x18 - 3k = x0

18 - 3k = 0 -3k = -18 k = 6

t6 + 1 = 18C6 218 - 6 (-1)6 x18 - 3(6)

Substitute k = 6:

t7 = 18C6 212 (-1)6

t7 = 76 038 144Therefore, the constantterm is 76 038 144.

Finding a Particular Term in a Binomial Expansion

x2 1x

10

.Find the numerical coefficient of the x11 term of the expansion of

tk + 1 = nCk an - k bk

n = 10a = x2

b = -x-1

k = ? tk + 1 = 10Ck (x2)10 - k (-x-1)k

tk + 1 = 10Ck x20 - 2k (-1)k x-k

tk + 1 = 10Ck(-1)k x20 - 2k x-k

tk + 1 = 10Ck (-1)k x20 - 3k

x20 - 3k = x11

20 - 3k = 11 -3k = -9 k = 3

t3 + 1 = 10C3 (-1)3 x20 - 3(3)

Substitute k = 3:

t4 = 10C3 (-1)3x11

t4 = -120 x11Therefore, the numericalcoefficient of the x11 term is -120.

One term in the expansion of (2x - m)7 is -15 120x4y3. Find m.

Finding a Particular Term of the Binomial

tk + 1 = nCk an - k bkn = 7

a = 2xb = -mk = 3

tk + 1 = 7C3 (2x)4 (-m)3

tk + 1 = (35) (16x4) (-m)3

-15 120x4y3 = (560x4) (-m)3

15 120x4 y3

560x4 m3

27y3 m3

27 y3 m3

27 y33 m

3y = m

Therefore, m is 3y.

The Binomial Theorem

I have made this one [letter] longer than usual because I did not have the leisure to make it shorter. 

— Blaise Pascal