1003112804

SubArea: 
Geometric sequences
Level: 
B
Project ID: 
1003112804
Accepted: 
1
Clonable: 
0
Easy: 
0
The third term of a geometric sequence is −5 and the eighth term is −5. s5 is the sum of the first five terms and q is the common ratio of this sequence. Choose the formula which is not valid for this sequence:
s5=−5⋅q5−1q−1
s5=−25
s5=5⋅a1
s5=5⋅a3
s5=5⋅(−5)