（1）那個(gè)問(wèn)題我沒(méi)有做錯(cuò)啊.是你的題目的選項(xiàng)設(shè)置有誤
1,cos(α-400°)cos(20°+α)-sin(α-40°)sin(200°+α)
=cos(α-40°-360°)cos(20°+α)-sin(α-40°)sin(180°+20°+α)
=cos(α-40°)cos(20°+α)+sin(α-40°)sin(20°+α)
=cos(α-40°-20°-α)
=cos(-60°)
=1/2
2. cosα=-3/5,α屬于(π/2,2π)?應(yīng)該是α屬于(π/2,π)吧,否則范圍就沒(méi)有意義了sinα>0又sin²α=1-cos²α=1-(-3/5)²=16/25∴ sinα=4/5
sinB=-12/13, B是第三象限角,則cosB=-5/13∴ cos(B-α)=cosBcosα+sinB*sinα=(-5/13)*(-3/5)+(-12/13)*(4/5)=-63/65