SOLUTION: Find t sub n if t sub 3 is equal to 15 and t sub 8 is equal to 50

, 

If the sequence is arithmetic, we use the formula:



three times. Once for n=3. once for n=8, and once using
the values for t1 and d.

--------------------------

Substitute n = 3





Substitute 15 for t3



-------------------------

Substitute n = 8





Substitute 50 for t8



So you have this system of equations to solve:



Solve that system by substitution or elimination and
get

 and 

---------------------------------

Therefore 



becomes









Edwin