cara ngitungnya gimana ya,, tolong

Optik Geometri.

M = -s' / s → s' = -Ms
r = 2f

1 / f = 1 / s + 1 / s'
f = ss' / (s + s')
r = -2Ms² / (s - Ms)
  = -2s / (1/M - 1)
  = 2s / (1 - 1/M)
  = 2(0,3 cm) / (1 - 1/5) = 0,75 m