Question

Find `a_1` if you are given `Sn=1800," "a_n=185 and n=18 ?`

Arithmethic Sequence

Answer 1

`a_1=15`

Explanation:

`"using the sum to n terms given the last term "a_n`

`•color(white)(x)S_n=n/2(a_1+a_n)`

`color(white)(S_n)=18/2(a_1+185)=1800`

`a_1+185=1800/9=200`

`a_1=200-185=15`