75,68,61,54 un-9 sn17

A = 75

b = 61 – 68

b = -7

Un = 75 – 7n + 7

Un = 82 – 7n

U9 = 82 – 7×9

U9 = 82 – 63

U9 = 19

U17 = U9 + (17-9)b

U17 = 19 + 8 x (-7)

U17 = 19 -56

U17 = -37

S17 = 17 x ( a + U17 )/2

S17 = 17 x ( 75 – 37)/2

S17 = 17 x 38/2

S17 = 17 x 19

S17 = 323