How is Pascal's triangle related to the Binomial Theorem?

Answer 1

I think an example will help most people see it better than just an explanation/answer. So first a few examples are presented and than a general answer.

Start with (1+x)2 = 1+2x+x2 and look at the coefficients of the results you will see that they are 1, 2, 1. Now do it for (1+x)3 and they are 1, 3, 3, 1. These, of course, are the lines from Pascal's Triangle. I put the first part of the triangle below (in left-justified form). You should notice that when the exponent is 2, we use the third line and when the exponent is 3, we use the fourth line of the triangle.

1

1 1

1 2 1

1 3 3 1

1 4 6 4 1

1 5 10 10 5 1

1 6 15 20 15 6 1

1 7 21 35 35 21 7 1

1 8 28 56 70 56 28 8 1

1 9 36 84 126 126 84 36 9 1

In case you wonder about the first two rows. Look at (1+x)0 and

(1+x)1, their coefficients come from those two rows.

Now look at (a + b)4 = a4 + 4a3b + 6a2b2 + 4ab3 + b4 which is more general and of course the coefficients come from the 5th line of the triangle.

So the answer to the question is that if we look at the binomial (a + b)n

The n+1 row of Pascal's triangle gives us the coefficients of the expanded form of the binomial. Seeing the examples first often makes this easier to see and understand. It is the n+1 row because the first row of the triangle is any binomial with an exponent of 0.