write down the main four formulas we ll be using on the test Explicit Formula for Arithmetic Sequences Explicit Formula for Geometric Sequences Period Partial Sum Formula for Arithmetic Series Partial Sum Formula for Geometric Series For the following sequences decide if it is arithmetic geometric or neither If it s arithmetic o geometric find the explicit formula Use it to find a 1 3 8 13 18 23 2 1000 500 250 125 67 5