sat2 Barron 英文的物理题

题目

sat2 Barron 英文的物理题
A 3-centimeter-tall candle is placed in front of a concave spherical mirror.An inverted image,half the size of the original,is observed on a screen.Where,relative to the mirror,must the candle have been placed?答案是beyond 2f.我想知道这道题里怎么用造镜者公式,就是1/di＋1/do＝1/f.

答案