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# Finding the sum of the first n terms of a geometric sequence.

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The sum of the first five terms of a geometric sequence is found. The solution is detailed and well presented. The response received a rating of "5/5" from the student who originally posted the question.

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## Geometric and Arithmetic Sequences

1) Use the arithmetic sequence of numbers 2, 4, 6, 8, 10... to find the following:
a) What is d, the difference between any 2 terms?
b) Using the formula for the nth term of an arithmetic sequence, what is 101st term?
c) Using the formula for the sum of an arithmetic series, what is the sum of the first 20 terms?
d) Using the formula for the sum of an arithmetic series, what is the sum of the first 30 terms?
e) What observation can you make about these sums of this series (HINT: It would be beneficial to find a few more sums like the sum of the first 2, then the first 3, etc.)?

2) Use the geometric sequence of numbers 1, 3, 9, 27, ... to find the following:
a) What is r, the ratio between 2 consecutive terms?
b) Using the formula for the nth term of a geometric sequence, what is the 10th term?
c) Using the formula for the sum of a geometric series, what is the sum of the first 10 terms?

3) Use the geometric sequence of numbers 1, 1/3, 1/9 , 1/27... to find the following:
a) What is r, the ratio between 2 consecutive terms?
b) Using the formula for the sum of the first n terms of a geometric series, what is the sum of the first 10 terms?
c) Using the formula for the sum of the first n terms of a geometric series, what is the sum of the first 12 terms?
d) What observation can make about these sums? In particular, what number does it appear that the sum will always be smaller than?

4) CLASSIC PROBLEM - A traveling salesman (selling shoes) stops at a farm in the Midwest. Before he could knock on the door, he noticed an old truck on fire. He rushed over and pulled a young lady out of the flaming truck. Farmer Crane came out and gratefully thanked the traveling salesman for saving his daughter's life. Mr. Crane insisted on giving the man an award for his heroism.

So, the salesman said, "If you insist, I do not want much. Get your checkerboard and place one grain of wheat on the first square. Then place two grains of wheat on the next square. Then place four grains on the third square. Continue this until all 64 squares are covered with grains of wheat." As he had just harvested his wheat, Mr. Crane did not consider this much of an award, but he soon realized he made a miscalculation on the amount of wheat involved.

a) How much wheat would Mr. Brown have to put on the 24nd square?
b) How much total grain would the traveling salesman receive if the checkerboard only had 24 squares?
c) Calculate the amount of wheat necessary to fill the whole checkerboard (64 squares). How much wheat would the farmer need to give the salesman?

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