Calculus

I have the question :A small blood vessel of radius 2mm branches off at an angle(theta) from a larger blood vessel of radius 4mm. According to Poiseuille's Law the total resistance to the blood flow is proportional to T=(a-bcot(theta)/4^4)+(bcsc(theta)/2^4) Show that the total resistance is minimized when cos(theta)=(1/16).
I think i need to find the derivative, which should be (bcsc^2(theta)/4^4)-(bcsc(theta)cot(theta)/2^4).
I have no clue if that is even correct, or what to do to solve this problem.

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  1. T=(a-bcot(theta)/4^4)+(bcsc(theta)/2^4)

    dT/dtheta =b times
    -1 (-csc^2)/4^4
    +1 (-csc ctn )/2^4

    set the derivative to zero for max or min

    csc^2/2^8 = csc ctn /2^4
    csc /ctn = 2^8/2^4 = 16

    1/sin / cos/sin = 16
    1/cos = 16
    cos = 1/16

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  2. patience, patience, you had it :)

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  3. I am so appreciative of your help, but what happened to the b?

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  4. b is a constant so
    when
    0 = b * (something - b * (something else)
    b cancels

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