Question

What is the equation for a geometric sequence with a first term of 4 and a second term of −8?

an = 4(2)n − 1 ?
an = 4(−2)n − 1 ?
an = 4(−12)n − 1 ?
or
an = 4(12)n − 1 ?

Answer 1

`a_n=4(-2)^(n-1)`

Explanation:

The common ratio is -8/4 = -2. The general formula for the n-th term is ...

... an = a1·r^(n-1) . . . . . . note the use of the caret (^) and parentheses

For r = -2 and a1 = 4, this becomes ...

... an = 4·(-2)^(n-1) . . . . approximately matches the 2nd selection