Question 1110215
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a) The numbers are the first 20 multiples of 3; so the number of terms in the sequence is 20.<br>
More formally...
{{{60-3 = 57}}}  difference between first and last terms
{{{57/3 = 19}}}  divide by common difference to find the number of terms AFTER THE FIRST TERM
{{{19+1 = 20}}}  you need to count the first term also<br>
b) Since this is an arithmetic sequence (common difference between successive terms), the average of all the numbers is the average of the first and last.<br>
Since in ANY set of numbers the sum is equal to the average multiplied by the number of numbers, the sum of the terms in this sequence is<br>
{{{((3+60)/2)*20 = 63*10 = 630}}}