SOLUTION: If A is in Quadrant III and {{{cot(a)=2}}} , find the value of {{{cos2a}}}

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Question 754380: If A is in Quadrant III and cot%28a%29=2 , find the value of cos2a
Answer by tommyt3rd(5050) About Me  (Show Source):
You can put this solution on YOUR website!
cot%28a%29=x%2Fy=2%2F1

so that
r=sqrt%281%5E2%2B2%5E2%29=sqrt%285%29

and so

cos%282a%29=2cos%5E2%28a%29-1=2%28x%2Fr%29%5E2-1=2%282%2Fsqrt%285%29%29%5E2-1=3%2F5