1a. Show that in any arithmetic sequence the n-th term in the sequence is equal to the average of the term before it and the term after it (except for the first term).

1b. Write the sum in sigma notation: 1-2x+3x²-4x³+5x⁴+...-100x⁹⁹

1c. rewrite the sum starting the index at i=1

i=515i2+11