1.a/sina=b/sinb=c/sinc=2r,sina=a/2r..a^2+b^2-c^2=ab,cosc=1/2,c=60或120
2.a+c=2b,cos60=1/2=a^2+b^2-c^2/2ac,a^2+c^2-((a+b)/2)^2=ac,a^2+c^2-2ac=0,a=c,又因为b=60度,所以abc为等边三角形
3.2sin(60-50)-sin50=根号3cosb-sin50+sin50/cosb=根号3
4.sn/n=(a1+an)*1/2,a1+a9/2-a1+a7/2=d=2,s10=(-10+8)*10/2=-10