cos2(-a)-tan(360+a)/sin(-a)

cos2(-a)-tan(360+a)/sin(-a)

cos2(-a)-tan(360+a)/sin(-a)=cos2a-tana/-sina=cos2a+1/cosa

If it belongs to (0, PAI), and cos □ + sin □ = - 1 / 3, then Cos2 □ =?

Let □ = a
cosa+sina=-1/3
∵a∈(0,π)
∴sina>0,
∴cosa

Given that α is the second quadrant angle, sin α + cos α = 1 / 2, then Cos2 α=

Square sin ^ 2 α + cos ^ 2 α + 2sinacosa = 1 / 4, and because sin ^ 2 α + cos ^ 2 α = 1, 2sinacosa = - 3 / 4, so (sin α - cos α) ^ 2 = 1 - (- 3 / 4) = 7 / 4, so (sin α - cos α) = 7 / 4 under the root sign, then Cos2 α = - (sin α + cos α) (sin α - cos α) = - 1 / 2 * under the root sign (7 / 4)