Sigma notation - Mathcentre

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Solution Notice that, in this example, there are 6 terms in the sum, because we have k = 0 for the first term: 5∑ 2 k = 2 0 + 2 1 + 2 2 + 2 3 + 2 4 + 2 5 Example 6∑ 1 Evaluate r(r + 1). 2 Solution r=1 k=0 = 1 + 2 + 4 + 8 + 16 + 32 = 63 . You might recognise that each number 1 r(r + 1) is a triangular number, and so this example 2 asks for the sum of the first six triangular numbers. We get 6∑ r=1 1 2 r(r + 1) = ( 1 2 × 1 × 2) + ( 1 2 × 2 × 3) + ( 1 2 × 3 × 4) + ( 1 2 × 4 × 5) + ( 1 2 × 5 × 6) + ( 1 2 × 6 × 7) = 1 + 3 + 6 + 10 + 15 + 21 = 56 . What would we do if we were asked to evaluate n∑ 2 k ? k=1 Now we know what this expression means, because it is the sum of all the terms 2 k where k takes the values from 1 to n, and so it is n∑ 2 k = 2 1 + 2 2 + 2 3 + 2 4 + . . . + 2 n . k=1 But we cannot give a numerical answer, as we do not know the value of the upper limit n. Example 4∑ Evaluate (−1) r . Solution r=1 Here, we need to remember that (−1) 2 = +1, (−1) 3 = −1, and so on. So 4∑ (−1) r = (−1) 1 + (−1) 2 + (−1) 3 + (−1) 4 r=1 = (−1) + 1 + (−1) + 1 = 0 . c○ mathcentre July 18, 2005 4
Example 3∑ Evaluate Solution k=1 ( − 1 k) 2 . Once again, we must remember how to deal with powers of −1: 3∑ k=1 ( − k) 1 2 = ( −1) 1 2 + ( −2) 1 2 + ( ) − 1 2 3 = 1 + 1 4 + 1 9 = 1 13 36 . 3. Writing a long sum in sigma notation Suppose that we are given a long sum and we want to express it in sigma notation. How should we do this? Let us take the two sums we started with. If we want to write the sum 1 + 2 + 3 + 4 + 5 in sigma notation, we notice that the general term is just k and that there are 5 terms, so we would write 5∑ 1 + 2 + 3 + 4 + 5 = k . To write the second sum k=1 1 + 4 + 9 + 16 + 25 + 36 in sigma notation, we notice that the general term is k 2 and that there are 6 terms, so we would write 6∑ 1 + 4 + 9 + 16 + 25 + 36 = k 2 . Example Write the sum in sigma notation. Solution k=1 −1 + 1 2 − 1 3 + 1 4 − . . . + 1 100 In this example, the first term −1 can also be written as a fraction − 1 . We also notice that the 1 signs of the terms alternate, with a minus sign for the odd-numbered terms and a plus sign for the even-numbered terms. So we can take care of the sign by using (−1) k , which is −1 when k is odd, and +1 when k is even. We can therefore write the sum as (−1) 1 1 1 + (−1)2 1 2 + (−1)3 1 3 + (−1)4 1 4 + . . . + (−1)100 1 100 . 5 c○ mathcentre July 18, 2005
Page 1 and 2: Sigma notation Sigma notation is a
Page 3: Exercises 1. Write out what is mean
Page 7 and 8: Suppose we have the sum of a consta
Page 9 and 10: Key Point If a and c are constants,

Sigma notation - Mathcentre

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