cosα=-√(1-16/25)=-3/5,
cos(α+β)=cosαcosβ-sinαsinβ=-(3/5)cosβ-(4/5)sinβ=(3/5)√(1-sin²β)-(4/5)sinβ=-5/13
于是得sinβ=0.8615或-0.2462.
cosα=-√(1-16/25)=-3/5,
cos(α+β)=cosαcosβ-sinαsinβ=-(3/5)cosβ-(4/5)sinβ=(3/5)√(1-sin²β)-(4/5)sinβ=-5/13
于是得sinβ=0.8615或-0.2462.