# Find a1 in the arithmetic series with s7=287 and d=12

Find a1 in the arithmetic series with s7=287 and d=12
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For an arithmetic series with initial term ${a}_{1}$ and difference between terms of d
$\text{XXX}{a}_{n}={a}_{1}+\left(n-1\right)d$

We are told $d=12$
and ${a}_{7}=287$

So
$\text{XXX}{a}_{7}=287={a}_{1}+\left(7-1\right)×12={a}_{1}+72$

$\text{XXX}\to {a}_{1}=287-72=215$