Question 1134390
<br>
Let a and d be the first term and common difference, respectively.  Then<br>
the 8th term is a+7d
the 20th term is a+19d<br>
The sum of the first 8 terms is
(a)+(a+d)+(a+2d)+...+(a+7d) = 8a+28d<br>
The sum of the first 20 terms is
(a)+(a+d)+(a+2d)+...+(a+19d) = 20a+190d<br>
So<br>
{{{8a+28d = 160}}}
{{{20a+190d = 880}}}<br>
{{{40a+140d = 800}}}
{{{40a+380d = 1760}}}
{{{240d = 960}}}
{{{d = 4}}}
{{{8a+112 = 160}}}
{{{8a = 48}}}
{{{a = 6}}}<br>
The first term is 6 and the common difference is 4.<br>
The 43rd term is a+42d = 6+42(4) = 6+168 = 174<br>
The sum of the first 12 terms is
(a)+(a+d)+(a+2d)+...+(a+11d) = 12a+66d = 12(6)+66(4) = 72+264 = 336<br>