Determine whether each series is arithmetic or geometric. Then find the sum of the given term.
a) 2 + 4 + 8 + 16 + . . . ; 10th term.
b) 2 + 4 + 6 + 8 + . . . ; 20th term

Question

Determine whether each series is arithmetic or geometric. Then find the sum of the given term.
a)	2 + 4 + 8 + 16 + . . . ; 10th term.
b)	2 + 4 + 6 + 8 + . . .  ; 20th term

anonymous · Answer

For the first one I got 

Geometric Sequence r=2
N=10, a1=2, r=2
An=a1*rn-1

A10=2*210-1

A10=2*512

A10=1024
 and the second one I got 

Arithmetic Sequence d=2

N=2, a1=2, d=2

An=a1+(n-1)d

A20=2+(20-1)2

A20=2+(19)2

A20=2+38

A20= 40

anonymous · Answer

Can any one tell me if these are both correct and if not why?

anonymous · Answer

i ll try..

anonymous · Answer

@boocoo I can vouch for you for the second one, it is indeed an Arithmetic sequence and I got A20 as 40 too. Oddly haven't done geometric sequences, so can't vouch for you there.

anonymous · Answer

The formula 4 a geometric sequence is a1 (r ^n-1) rite? i cnt remember..

anonymous · Answer

yes that is the right formula.

anonymous · Answer

2 (2^10-1) 
2(2^9) 
1024

yeah ur first 1 is rite

anonymous · Answer

ok thanks :)