Need Help

[ttb]

1+2+3+4=10. It there a way with knowing the number 4 that your answer is 10? Another example 1+2+3+4+5+6+7=28. With knowing the number 7 can I determine the answer is 28?
I have several stock trades that I make, first I buy one share then two then three then four. Having bought the fourth group of shares I then have a total of 10 shares. Now imagine I have done this 10,000 times. How many share do I now have?
Thanks for your time.

 

[blamocur]

Search for "arithmetic progression sum" on internet. Some of the links:

• https://en.wikipedia.org/wiki/Arithmetic_progression#Sum
• https://www.khanacademy.org/math/pr...:arith-series/v/formula-for-arithmetic-series
• https://www.varsitytutors.com/hotmath/hotmath_help/topics/arithmetic-series

 

[ttb]

Search for "arithmetic progression sum" on internet. Some of the links:

• https://en.wikipedia.org/wiki/Arithmetic_progression#Sum
• https://www.khanacademy.org/math/pr...:arith-series/v/formula-for-arithmetic-series
• https://www.varsitytutors.com/hotmath/hotmath_help/topics/arithmetic-series

Thank you. I wouldn't have even known what it was called.

 

[ttb]

Thank you. I wouldn't have even known what it was called.

Got it! How neat! Thanks again.

 

[Steven G]

1 + 2 + 3 + ... + n = n(n+1)/2. So yes, you can find the sum on the left side by just knowing the last number.