1. ## Arithmetic Sequence problem

The sum of the fifth term and twice the third term of an arithmetic sequence equals the twelfth term. If the seventh term is 25, find an expression for the general term $u_n$

2. Originally Posted by shawli
The sum of the fifth term and twice the third term of an arithmetic sequence equals the twelfth term. If the seventh term is 25, find an expression for the general term $u_n$
$U_5 + 2U_3 = U_{12}$

Since $U_n = a + (n-1)d$ we can rewrite this as

$(a+4d) + (2a+4d) = a + 11d$

You can simplify that down by collecting like terms.

$U_7 = a + 6d = 25$

You know have two equations from which you can find a and d

3. Errr nevermind, I got it!

Thanks for clicking though