In triangle XYZ, we have angle X = 90 and tan Z = 3. What is cos Z?
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Buiding the right triangle, you can get that cos(Z) = 2*sqrt(2)/3.
For simplicity, let's just sat ZX = 1 and XY = 3, and that would make ZY = sqrt(10).
Cos Z = ZX/ZY
cos z = 1/sqrt(10) = sqrt(10)/10.
I hope this helped. :))