三角形面积S=a·1/2=b·3/2=c·5/2
则三边之比为a:b:c=15:5:3
a=2S;b=(2/3)S;c=(2/5)S.
根据余弦定理,有
cosA=(b^2+c^2-a^2)/2bc
=-89/180.
则sinA=√(1-cos^2 A)=0.869
则三角形面积为
S=(1/2)bc·sinA
=(2/15)S^2 ×0.869
=0.1159·S^2
则S=1/0.1159=8.629