Use sigma notation to write the sum
20 + 25 + 30 + 35 + ... + 100.
All the terms here are multiples of 5.
20 + 25 + 30 + 35 + ... + 100 = 5(4) + 5(5) + ... + 5(20).
If the general term is 5n, then the series should start at n = 4 and stop at n = 20.
Write the sum
4(0.1) + 4(0.01) + 4(0.001) +...
using summation notation.
The nth term is
where we start at n = 1, so we write
Use sigma notation to write the series
starting at (a) n = 3, (b) n = 2, (c) n = 1 and (c) n = 0.
(a) We can rewrite the series so the terms are fractions whose denominators are powers of 2:
From here it makes sense to view the series as starting at n = 3 and having general term
Then we can write the series in sigma notation as
(b) Way 1: We already saw that we can rewrite the series as
In order to think of the first term as using n = 2 instead of n = 3, we can break up the denominators like this:
Then we can write the series as
Way 2: Pull out from each term and rewrite the series as
This can be viewed as a series with general term
starting at n = 2. We write the series in summation notation as
This is equivalent to what we got with Way 1.
(c) Way 1: In order to start at n = 1, we can break up the denominators like this:
Way 2: This is very similar to (b), except that we'll pull out from each term so we can view the remaining powers of 2 as starting at n = 1.
(d) Way 1: Rewrite the series so the denominator of term an has a factor of 2n, starting at n = 0:
Way 2: If we want the first term to involve 2 with an exponent of 0, we need to factor out from every term:
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