the binomial theorem 1 objectives: pascal’s triangle 1 1 2 1 1 3 3 1 coefficient of (x + y) n when...
TRANSCRIPT
The binomial theorem 1
Objectives:
Pascal’s triangle
1 1
1 2 1 1 3 3
1
Coefficient of (x + y)n when n is large
Notation: )( rn ncr
Expansion of (x + y)n for n = 2, 3 and 4
(x + y)2 = x(x + y) + y(x + y) = x2 + 2xy + y2
(x + y)3 = (x + y)(x + y)2 = = x(x2 + 2xy + y2) + y(x2 + 2xy + y2)
(x + y)(x2 + 2xy + y2)
= x3 + 2x2y + xy2
+ x2y + 2xy2 + y3
= x3 + 3x2y + 3xy2 + y3
(x + y)4 = (x + y)(x + y)3 =
= x(x3 + 3x2y + 3xy2 + y3) + y(x3 + 3x2y + 3xy2 + y3)
= x4 + 3x3y + 3x2y2 + xy3
= x4 + 4x3y + 6x2y2 + 4xy3 + y4
x3 + 3x2y + 3xy2 + y3
+ x3y + 3x3y2 + 3x2y3 + y4
Expansion of (x + y)n
(x + y)2 = 1x2 + 2xy + 1y2
(x + y)3 = 1x3 + 3x2y + 3xy2 + 1y3
(x + y)4 = 1x4 + 4x3y + 6x2y2 + 4xy3 + 1y4
(x + y)1 = 1x + 1y
(x + y)5 = 1x5 + 5x4y + 10x3y2 + 10x2y3 + 5xy4 + 1y5
(1 + y)5 = 1(1)5 + 5(1)4y + 10(1)3y2 + 10(1)2y3 + 5(1)y4 + 1y5
= 1 + 5y + 10y2 + 10y3 + 5y4 + y5
Examples: Write down the expansions:
(x + y)2 = 1x2 + 2xy + 1y2
(x + y)3 = 1x3 + 3x2y + 3xy2 + 1y3
(x + 3y)2 =
(x + 3y)2 = 1x2 + 2x(3y) + 1(3y)2 = x2 + 6xy + 9y2
(4 + y)3 =
(4 + y)3 = 1(4)3 + 3(4)2y + 3(4)y2 + 1y3 = 64 + 48y + 12y2 + y3
(x + y)3 = 1x3 + 3x2y + 3xy2 + 1y3
(2a + 3b)3 =
(2a + 3b)3 = 1(2a)3 + 3(2a)23b + 3(2a)(3b)2 + 1(3b)3
= 8a3 + 36a2b + 54ab2 + 27b3
Examples:
Write the coefficient of x3 in the expansion of (2x- 3)4.
(x + y)4 = 1x4 + 4x3y + 6x2y2 + 4xy3 + 1y4
x3 is the 2nd term : 4(2x)3(-3) = 4(8x3)(-3) = -96x3
The required coefficient is - 96
In the expansion of (1 + bx)4, the coefficient of x3 is 1372. Find the constant b.
y3 is the 4th term : 4(1)(bx)3 = 4b3x3 = 1372
4b3 = 1372
(x + y)4 = 1x4 + 4x3y + 6x2y2 + 4xy3 + 1y4
b3 = 343
b = 7