`cos u = -4/5, pi/2

By the double angle formulas

`cos(2u)=2cos^2u-1,`

`sin(2u)=2sin(u)cos(u).`

By the first formula `cos(2u)=2*(-4/5)^2-1=7/25.`

Because `u` is in the second quadrant, its sinus is positive and is equal to

`sqrt(1-cos^2u)=3/5.`

So by the second formula `sin(2u)=2*3/5*(-4/5)=-24/25.`

And `tan(u)=sin(u)/cos(u)=-24/7.`

The answer: `sin(2u)=-24/25,` `cos(2u)=7/25,` `tan(2u)=-24/7.`