Question

Write the explicit formula for the geometric sequence.

a1 = -5 a2 = 20 a3 = -80

Answer 1

\[U_n = U_1r^{n-1}\]

where r is the common ratio. use this formula on two of the above, and solve for U_1 and r

Answer 2

although we know U_1 = a_1

Answer 3

Write the explicit formula for the geometric sequence.

a1 = -5 a2 = 20 a3 = -80

A. an = -5 • (-4)n

B. an = -5(-4)n-1

C. an = -4(-5)n-1

D. an = -5 • (4)n

Answer 4

\[a_2 = ra_1 \text{ so } 20 = -5r \]

r = -4

so the general formula is

\[a_n = -5(-4)^{n-1}\]