How do you expand #(1+x^3)^4# using Pascal’s Triangle?

1 Answer
Feb 8, 2016

Since there are (4 + 1) = 5 terms in this expansion, we must find the numbers located in the #5^(th)# term of the Pascal's Triangle. To find the number of terms in an expansion, always add 1 to the exponent, as to include the #0^(th)# term.


Draw a diagram to represent Pascal's Triangle. Each row is the sum of the numbers above it, with 1 at the first row, (1 and 1) at the second row, (1, 2 and 1) in the third row. The following diagram is of Pascal's Triangle:

Counting up from the row with a single 1, we find that row 5 contains the numbers 1, 4, 6, 4 and 1.

To expand, the exponents on the 1 will start at 4 and will decrease until 0. The exponents on the #x^3# will increase from 0 to 4. As you can see, in each term the exponents must add up to the expression's exponent, which in this case is 4.

#1(1)^4(x^3)^0 + 4(1)^3(x^3)^1 + 6(1)^2(x^3)^2 + 4(1)^1(x^3)^3 + 1(1)^0(x^3)^4#

Simplifying by using exponent laws:

#1 + 4x^3 + 6x^6 + 4x^9 + x^12#

When fully expanded, #(1 + x^3)^4# = #1 + 4x^3 + 6x^6 + 4x^9 + x^12#. As you can see, in each t

Practice Exercises:

  1. Expand #(2x - 3y)^5# using Pascal's Triangle.

  2. Find the 3rd term in #(x + 3)^7#. Hint: Think of finding the appropriate number in the Pascal's Triangle and plugging it in for nCr in #t_(r + 1) = nCr(a)^(n - r) xx b^r#.

Good luck!