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Calculus help - Find the sensitivity?

If R denotes the reaction of the body to some stimulus of strength x , the \ sensitivity S is defined to be the rate of change of the reaction with respect to x . A particular example is that when the brightness x of a light source is increased, the eye reacts by decreasing the area R of the pupil. The experimental formula R= (35+16x^4)/(1+0.5x^4)

can be used to model the dependence of R on x when R is measured in square millimeters and x is measured in appropriate units of brightness.

Find the sensitivity corresponding to x=2

Sensitivity=?????

1 Answer

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  • 1 decade ago
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    In short, the sensitivity is the derivative of R.

    R= (35 + 16x^4) / (1 + 0.5x^4)

    Using the quotient rule, we have:

    R'

    = [(1 + 0.5x^4)(64x^3) - (35 + 16x^4)(2x^3)] / (1 + 0.5x^4)^2

    = [64x^3 + 32x^7 - 70x^3 - 32x^7] / (1 + 0.5x^4)^2

    = [-6x^3] / (1 + 0.5x^4)^2

    When x = 2, we have:

    sensitivity

    = R'(2)

    = [-6(2)^3] / (1 + 0.5(2^4))^2

    = -48 / 9^2

    = -48 / 81

    = -16/27

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