Here you can keep your brain fit by solving math related problems and also earn SBI by doing so.
The problems usually contain a mathematical equation that in my opinion is fun to solve or has an interesting solution.
I will also only choose problems that can be solved without additional tools(at least not if you can calculate basic stuff in your head), so don't grab your calculator, you won't need it.

Thanks to @tonimontana there is a 2 SBI giveaway for this contest.
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Every friday and tuesday!

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Rules

No upvote, No resteem, No follow required!

I will give 1 SBI to 2 random persons who put a correct solution of the problem in a comment.

You have 48 hours to solve it.

Some additional rules for the case that the problem is an equation or a collection of equations.

In case there are multiple solutions, you have a higher chance of winning if you get all of them correctly.

Solutions can sometimes also be general like x = 20, y > -1. If there are multiple solutions submitted, that are part of the same general solution, they will be counted as one solution.

And one more thing: I only want integer solutions!

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Problem

Since I'm out of equations for now, there is something different today consisting of 2 steps:

1. Find the n-th prime, where n is your reputation(the number displayed behind your username).
2. Sum up all integers between (and including) 1 and your prime.

Also please write down your reputation.

A small example(so you all understand):

sampleuser(3):
"My reputation is 3.
The 3. prime is 5.
1+2+3+4+5 = 15"

You think this is unfair for people with a high reputation?
Only a little bit. Finding this sum is easier then just adding all the terms manually.

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You can take a look at the results of the last contest here. There you can also find out how I evaluate your entries.

To everyone who already participated in a past contest, come back today and try a new problem(tell me if you don't want to be tagged):
@addax @ajayyy @bwar @contrabourdon @heraclio @hokkaido @mytechtrail @onecent @sidekickmatt @sparkesy43 @tonimontana @vote-transfer

In case no one gets a result(which I doubt), I will give away the prize to anyone who comments.

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My announcements don't seem to last long… My last one was reverted by @contrabourdon (by sponsoring this contest) before I was able to actually implement it.
But here you get it anyway:

Announcement

From now on there will be 2 contests a week(starting on friday and tuesday).
The topics will sometimes involve more than just equations(like here).
The rewards will be higher on average.

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@contrabourdon sponsors this contest with 2 STEEM weekly.
You can support him by using a witness vote on untersatz, so he can further support this and other contests.