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So, i got this new phone (samsung gravity touch) and there a lot of apps for social networking and stuff (like twitter and facebook). When i go on these it says additional charges may be added, but when i click on web browsing in general it doesn't say anything. Is this free or do i have to pay...

For example how would you derive y = MN/P, where M, N and P are real numbers and P cannot equal 0.