- #1

- 46

- 0

Find the indicated sum of each arithmetic series

S15 for 4 + 11 + 18 + ...

Sn= n/2 [ 2a + ( n - 1 ) d ]

Sn= 15/2 [ 2(4) + (15 - 1 ) 7 ]

Sn= 7.5 [ 8 + 98 ]

Sn=

__795__

Find t8 for the following sequence

10 000 , -5 000 , 2 500 , -1 250 ,

So...

tn= ar ^ n-1

tn= 10 000 (-0.5) ^ n-1

t8= 10 000 (-0.5) ^ 7

=

__78.125__

Sometimes the answers in the back of the book are wrong, so am I wrong or are they??