Factorial problem problems!?

Question

So I have these two problems on my homework:

1+2+...+80

and

1+2+...+120?

I need help, like how to solve them, if you want to, you can but you don't have to solve them.

Answer 1

The key to doing these problems is using the equation:

1+2+3+...+n = n*(n+1)/2.

In the first case, n = 80, so the sum is equal to 80(81)/2 = 3240.

In the second problem, n = 120, so the sum is equal to 120(121)/2 = 7260.

By the way, these aren't factorial problems.  A factorial problem would be 7! = 7*6*5*4*3*2*1

Answer 2

Those aren't factorials. Factorials take the form of 1*2*3*..*n where that is represented as n!. The sum of the first n consecutive numbers is found with the function n(n+1)/2. If you're curious, you can find a proof of this on wikipedia.

For the first one, n = 80. 80(80+1)\2 = 40*81 = 3240.

With that knowledge, I'm sure you can do the 2nd yourself.