A voltage divider allows you to get a reduced voltage from a battery with higher voltage. For this circuit Vi = 5V, R1 = 1kohm, R2 = 9kohhm. Vo is the desired output voltage presented on R2.
Vo = Vi x (R2 / (R1 + R2))
Vo = 5 x (9k / (1k + 9k))
Vo = 5 x (9k / 10k)
Vo = 5 x (0.9)
Vo = 4.5V
