Lens-Maker Formula: A double-convex lens is formed from a piece of plastic with an index of refraction of 1.70. The radius of the front surface has magnitude 20 cm, and the radius of the back surface has magnitude 30 cm. What is the focal length of this lens?
A. 17 cm
B. 86 cm
C. -86 cm
D. -17 cm
E. 43 cm
Answer: A
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