A dentist's mirror has a radius of curvature of 3 cm. How far must it be placed from a small dental cavity to give a virtual image of the cavity that is magnified five times ?

Given ;
R = -3 cm
f = R/2 = -1.5 cm
m = 5
m = -v/u
5 = -v/u
5u = -v
v = -5u
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Using mirror formula;

1/f = (1/v)+(1/u)
(-1/1.5) = (-1/5u) + (1/u)
(-10/15) = (-1+5)/5u
-2/3 = 4/5u
-10u = 12
u = -12/10
u = -1.2 cm
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Hence the mirror should be placed 1.2cm away from the dental cavity to produce 5 times magnification