f1=20 cm = lentila convergenta
f2=-5cm= lentila divergenta
[tex]c 1= \frac{1}{f1} = \frac{1}{20} = 0.05cm = 0.5d[/tex]
[tex]c2 = \frac{1}{f2} = \frac{1}{ - 5} = - 0.2 = - 2d [/tex]
2d>0.5d
b) C=C1+C2
C= 0.5-2
C=-1.5D
