11. Find the sum of the first n odd numbers.
12. An arithmetical progression consists of 21 terms. The sum of the three terms in the middle is 129; the sum of the last three terms is 237. Find the series. (Look up the short method for such problems.) (Mass. Inst. of Technology.)
13. B travels 3 miles the first day, 7 miles the second day, 11 miles the third day, etc. In how many days will B overtake A who started from the same point 8 days in advance and who travels uniformly 15 miles a day?
Reference: The chapter on Arithmetical Progression in any algebra.
GEOMETRICAL PROGRESSION
1. Define a geometrical progression.
Learn to derive the four formulas in geometrical progression:
