底角A 顶角C
sin=120/169
cos=-119/169
tan=-120/119
sinA = 5/13 设顶角为A,底角为B,C
sinB=5/13
cosB=(1-(5/13)^2)^(1/2)=12/13
sin(A/2)=cosB=12/13
cos(A/2)=sinB=5/13
cos(2B)=2(cosB)^2-1=(2*12^2-13^2)/13^2>0
2B90度
sinA=2sin(A/2)cos(A/2)=120/169
cosA=-(1-(sinA)^2)^(1/2)=-7/13
tgA=sinA/cosA=-120/91
cosA = 12/13
sin(C/2) = 12/13
sinC = 2sin(C/2)cos(C/2) = 120/169