1. ## sum of integers

Find the sum of the first 100 positive even integers. Explain how this sum can be found quickly.
Sn = (n/2)(a1 + an)
is that the correct formula?

2. Originally Posted by aikenfan
Find the sum of the first 100 positive even integers. Explain how this sum can be found quickly.
Sn = (n/2)(a1 + an)
is that the correct formula?
that is the correct formula. note that we can consider an ordered list of even integers as an arithmetic sequence. we use that formula to find the sum of the terms of an arithmetic sequence. you know that $\displaystyle a_1 = 2$, and we want $\displaystyle n = 100$, so now the only thing left for you to find is $\displaystyle a_100$

3. Hello, aikenfan!

Find the sum of the first 100 positive even integers.
Explain how this sum can be found quickly.

Sn = (n/2)(a1 + an)
is that the correct formula?
That formula will work, but I prefer the general Sum Formula:

. . $\displaystyle S_n \;=\;\frac{n}{2}\left[2a_1 + (n-1)d\right]$

It is more complicated, but we don't need to calculate the 100th term.

We have: .$\displaystyle a_1 = 2,\;d = 2,\;n = 100$

Therefore: .$\displaystyle S_{100} \;=\;\frac{100}{2}\left[2\!\cdot\!2 + (100-1)2\right] \;=\; 10,100$

4. I am not sure how to calculate the 100th term?
an = dn + c?

5. Originally Posted by aikenfan
I am not sure how to calculate the 100th term?
an = dn + c?
you would use the formula $\displaystyle a_n = a_1 + (n - 1)d$ (which is the general formula for the nth term of an arithmetic sequence) where $\displaystyle a_n$ is the nth term, $\displaystyle a_1$ is the first term, $\displaystyle n$ is the current number of the term, and $\displaystyle d$ is the common difference. you would use the values Soroban gave

thus to find $\displaystyle a_{100}$ you would compute $\displaystyle a_{100} = 2 + (100 - 1)(2)$

6. an = dn + c

Isn't this the same thing? I get 200 either way...

7. Originally Posted by aikenfan
an = dn + c

Isn't this the same thing? I get 200 either way...
well, if you define c as a_1 - d, then yes. but i've never seen anyone use "c" in these formulas

8. Sn = (100/2)(2+200) = 10100