Question 1141684
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(1) The sum of 21 terms of an AP is 21 times the average of the first and last.  Let the first term be x; then<br>
{{{189/2 = 21((x+11/2)/2)}}}<br>
Solve to find the first term.<br>
(2) The 21st term is the first term plus the common difference d 20 times.  Use the given 21st term and the first term from (1) above to find the common difference d.<br>
(3) Use the common difference and the first term to find the second and third terms.<br>
(4) Add the first three terms to get the answer to the problem.