Help 17,18 ​

17

5th term  = 140

7th term = 35

We have that  

(140)r^2  = (35)       where r  is the common ratio

Divide both sides by  140

r^2  = 35/140  =  1/4        take both roots

r = +/-  sqrt  (1/4)   =  +/- 1/2

So....note that the 6th term could be  (140)(1/2)  = 70

Or.....the 6th term could be (140) (-1/2)  =  -70

Thus.....  ± 70   is correct

18

Note that the  first term  is 2(1) + 1 =  3   and the last term  is 2(12) + 1  = 25

So...the sum is

[ first term + last term ]  *  [ number of terms ] / 2  =

[ 3 + 25 ] *  [ 12]  / 2  =

28 *  6  =

168

The Answer is 168.