How do you calculate this? The sum of $4$ terms of an arithmetic sequence is $42$ and the sum of the $3^{rd}$ and $7^{th}$ term is $46$. How do you find the sum of $20$ terms?
$Sn = n/2 [2a+d(n−1)]$ is the equation for working out the sum of an arithmetic series, but how can i rearrange to find for the first term and common difference. I belive it would be using simulatenous equations.