急!F6 Geometric Sequences 02ex3

10.
a + (a+d) + (a+2d) = 3a + 3d = 9

[(a+3d) + (a+6d)]*4/2 = (2a+9d)*2 = -30

{ a + d = 3
{ 2a + 9d = -15

7d = -21
d = -3
a = 6

The first term = 6


17.
I. x^4 - x^2 can be different from x^6 - x^4

II.
(2x-1) - (x-2) = x + 1
3x - (2x-1) = x + 1
(4x+1) - 3x = x + 1
The differences are common, so it is an arithmetic sequence.

III.
log x, 2 log x, 3 log x, 4 log x clearly has a common difference of log x
The differences are common, so it is an arithmetic sequence.

II and III are A.S.
The answer is C.