If sin β = - 5 / 13, α∈ (π / 2, π), then Cos2 α=

If sin β = - 5 / 13, α∈ (π / 2, π), then Cos2 α=

Cos2 α = 1-2sin square α
=1-2 × (- 5 / 13) square
=1-2×25/169
=1-50/169
=-144/169