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Question

For a 35mm camera the equation 1/f=1/di+1/do represents the focal length (f) of the lens that is needed to produce a clear photograph. The distance from the lens to the film di mist be adjusted according to the distance from the lens to the object do if di=x^2-4x-12 and do= 4x+8, what is 1/f?

Answer 1

1/f = 1/[x^2-4x-12] + 1/(4x+8)

1/f = 1/[(x-6)(x+2)] + 1/[4(x+2)]

1/f = [4+(x-6)]] / [4(x-6)(x+2)]

1/f = (x-2) / [4(x-6)(x+2)]