HELP PLS Find the formula for the nth term of the sequence below.

20, −10, 5, −2.5, ...

a)an=−12⋅20nA sub n is equal to negative 1 half times 20 to the n th power

b)an=−12⋅20n−1A sub n is equal to negative 1 half times 20 raised to the n minus 1 power

c)an=20⋅(−12)nA sub n is equal to 20 times open paren negative 1 half close paren to the n th power

d)an=20⋅(−12)n−1