Find the number of terms in the arithmetic sequence whose first term is 11 and last term is 81, where the sum of all the terms is 506.
Find the number of terms for which the sum of the sequence is 0.
An employee’s annual salary in his first year was 6 500 LE. Every year his salary exceeded that of the year before by 240 LE. Find after how many years his annual salary would be 8 420 LE, and then find out the total salary he received during this period.