OK, these questions are supposed to be really easy, but I keep geting the wrong answer and I don't understand why, could someone pleas look at my work and tell me what I am doing wrong?(adsbygoogle = window.adsbygoogle || []).push({});

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??

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# Homework Help: Arithmetic series

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