Jawabanmu

2014-05-30T18:07:49+07:00
S = jarak benda
S' = jarak bayangan

 \frac{1}{ s^{'} } +  \frac{1}{s} =  \frac{1}{f}

 \frac{1}{20} +  \frac{1}{80} =  \frac{1}{f}

 \frac{4+1}{80}  =  \frac{1}{f}

 \frac{5}{80} =  \frac{1}{f}

f =  \frac{80}{5} = 16cm
2014-05-30T18:11:20+07:00
F = s x s'
     s + s
  = 20 x 80
     20 + 80
  = 1600  = 16 cm
     100