若sinβ=-5/13,α∈(π/2,π)则cos2α=

问题描述:

若sinβ=-5/13,α∈(π/2,π)则cos2α=

cos2α=1-2sin平方α
=1-2×(-5/13)平方
=1-2×25/169
=1-50/169
=-144/169