Question

An arithmetic progression has first term 11 and Fourth term 32. Fine the sum of the first nine term

Answer 1

Answer:351

Step-by-step explanation:Try to plus the same number every time, I used 7 and it worked. : )

Answer 2

Let the first term be a, 4th term be $a_{4}$, difference between terms be d, and sum of first nine terms be $S_{9}$.

$a_{4}$ = a + 3d

32 = 11 + 3d

3d = 32 - 11

3d = 21

d = 7

$S_{9}$ = $\frac{9}{2} (2a + (9-1)d)$

   = $\frac{9}{2} (22+56)$

   = 9(11+28)

   = 9(39)

   = 325

Sum of first nine terms is 351.