Question

How do you find a1 in the arithmetic series with s7=287 and d=12?

Answer 1

`a_1=215`

Explanation:

For an arithmetic series with initial term `a_1` and difference between terms of `d`
`color(white)("XXX")a_n=a_1+(n-1)d`

We are told `d=12`
and`a_7 = 287`

So
`color(white)("XXX")a_7 = color(blue)(287) = a_1+(7-1)xx12 = color(blue)(a_1+72)`

`color(white)("XXX")rarr a_1= 287-72 = 215`