Jawabanmu

2014-06-18T20:30:12+07:00
S= 4cm
M= 5x

M= s' / s 
5 = s' / 4
s' = 5 * 4 = 20 cm.

1/f = 1/s + 1/s'
     = 1/4 + 1/20
     = 5/20 + 1/20
     = 6/20
f= 20/6 = 3,33 cm.

maaf kalau salah.
2014-06-18T20:30:42+07:00
M =  \frac{S'}{S}
5   =  \frac{S'}{4}
S' = 20

1/f = 1/s + 1/s'
1/f = 1/4 + 1/20
f = 20/6