Question 839181
find cos x , if tan 2x=-3/4
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cos(2x)=-4/5
{{{cos(x)=cos(2x/2)=sqrt((cos(2x)+1)/2)=sqrt(((-4/5)+1)/2)=sqrt((1/5)/2)=sqrt((1/5)/2)=sqrt(1/10)=sqrt(10)/10}}}

calculator check:
tan 2x=-3/4
2x&#8776;143.13&#730;(in quadrant II in which tan<0)
x&#8776;71.56&#730;
..
cosx as calculated=&#8730;10/10
x&#8776;71.56&#730;