Need help plugging in terms for arithmetic problem

Each level of a smartphone app adds more gems for you to match. On level one, there were 13 gems. On level twelve, there were 123 gems. When you complete all twelve levels, how many total gems will you have matched?

605

660

748

816

Question

Need help plugging in terms for arithmetic problem

Each level of a smartphone app adds more gems for you to match. On level one, there were 13 gems. On level twelve, there were 123 gems. When you complete all twelve levels, how many total gems will you have matched?

605

660

748

816

anonymous · Answer

i know n=nth = 12
          a1=value of 1st term = 13
          a2= value of nth term = 123

anonymous · Answer

Level   : 1....2...3...4...5...6...7...8...9...10...11...12
Gems  : 13                                                         123
We have to increase from 13 to 123 over a span of 11 levels.
That's an increase of 110 over 11 levels. 110 / 11 is 10
So we go up 10 gems each level     
Level   : 1......2.....3.....4.....5.....6.....7.....8.....9......10......11......12
Gems  : 13    23   33   43  53   63  73     83   93  103     113     123

anonymous · Answer

Add up all those gems and you'll have the answer

anonymous · Answer

ty

anonymous · Answer

no problem