sin(2u) = 2*sin(u)*cos(u), cos(2u) = 2*(cos(u))^2-1, tan(2u) = sin(2u)/cos(2u).

So we have to find sin(u). sin(u) = +- sqrt(1-(cos(u))^2) = +-sqrt(5/9) = +-sqrt(5)/3.Choose "+" for sin(u) for pi/2 < u < pi.

So sin(2u) = 2*(sqrt(5)/3)*(-2/3) = -(4/9)*sqrt(5),cos(2u) = 2*(4/9) - 1 = -1/9 andtan(2u) = 4*sqrt(5).

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