【評論主題】17 一個薄凹透鏡在空氣中，物體位於透鏡第一焦點左側 30 cm 處，其像位於凹透鏡第二焦點右側 13.33 cm處，則透鏡的屈光度為何？ (A)-3.00 D (B)-3.33 D (C)-5.00

【評論內容】已知 (1/像距)+(1/物距)=1/焦距1/(f-13.33) + 1/(f-30) =1/f=>通分*(f-13.33)(f-30)=>(f-30)+(f-13.33)=(f-30)(f-13.33)/f=>f(f-30)+(f-13.33)=(f-30)(f-13.33)=>2f2-43.33f=f2-43.44f+30*13.33=>f2=30*13.33=399.9=>f=20因透鏡為凹透鏡F=1/-0.2=-5D