Question 921232
sin u = 20/29, cos u = -21/29  {{{sqrt(29^2 - 20^2 )= 21}}}
cos v = -8/17, sin v = 15/17  {{{sqrt(17^2 - 8^2 )= 15}}}
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csc(u - v) = 1 / sin(u - v)   =  1/(sin u cos v - cos u sin v)
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