A 12.0 cm object is 9.0 cm from a convex mirror that has a focal length of -4.5 cm. What is the distance of the image from the mirror?

Use the mirror equation, 1/f = 1/d_o + 1/d_i. With f = −4.5 cm and d_o = 9.0 cm, 1/d_i = 1/(−4.5) − 1/9.0 = −2/9 − 1/9 = −1/3, so d_i = −3 cm. The image is 3 cm behind the mirror, so the correct choice is “−3 cm.”

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