90.
C=1/f=(n-1)(1/R₁-1/R₂)
R₁=3 m;R₂=-6 m
f=R₁R₂/[(n-1)(R₂-R₁)]=3*(-6)/[(1,5-1)(-6-3)]=-18/0,5*(-9)=2/0,5=4 m=>f=4 m
91.
C=1/f=>f=1/C=1/2=0,5 m
R₁=-R₂=|R|
C=1/f=(n-1)(1/R₁-1/R₂)=2(n-1)/|R|=>|R|=2f(n-1)
C'=(n/n'-1)(1/R₁-1/R₂)=1/f'=>f'=fn'(n-1)/(n-n')=0,5*1,75(1,5-1)/(1,5-1,75)=0,43/(-0,25)=-1,75 m=>f'=-1,75 m
