α∈(π/2,π),且sinα=3/5cosα=-4/5cos(α-π/4) =cosαcosπ/4+sinαsinπ/4=-4/5*√2/2+3/5*√2/2=-4√2/10+3√2/10=-√2/10α∈(π/2,π)α/2∈(π/4,π/2)cosα=-4/5cosα=1-2sin²α/2-4/5=1-2sin²...谢谢。sin2α/2=(1-cosα)/2