Originally Posted by

**selin** In an arithmetic sequence, the fifth is 21 and the eighth is 3.

Determine

a) the first number of the sequence

b) the sum of the sixteen first numbers.

a) I got the answer to 45. So a1=45

But I can't solve b, because I get a completely different answer than what I should get. (the answer should be 0)

I thought:

n=16

a1=45

d=-6

an=a1+(n-1)*d

an=45+(16-1)*(-6)

an=45-90

an=-45

sum=(n/2)*(a1+an)

sum=(16/2)*(16+(-45))

**a1 is not 16, should be (16/2)*[45 + (-45)]**

sum=8*(-29)= -232

so I got the sum to -232, but it should be 0. How did I think wrong?