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LV超3A名牌購物網
The image of the "test chart" is 3 m behind the mirror. Because 小華 is x m in fromt of the mirror, and to maintain a standard distance of 5 m between 小華 and the chart, we have
x + 3 = 5
i.e. x = 2 m
D = 100/f
where D = 500 units of short-sightedness
f is the focal length (in m) of the lens
hence, f = 100/D = 100/500 m = 0.2 m = 20 cm
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