11.a∥b→1=3sin²θ
cos2θ=1-2sin²θ=1-2/3=1/3
12.设sinA=3m,sinB=2m,sinC=4m,m≠0
a/sinA=b/sinB=c/sinC=n,n≠0,
则a=nsinA=3mn,b=2mn,c=4mn,mn≠0
c²=a²+b²-2abcosC
16=9+4-12cosC
cosC=-1/4
13.y‘=3ax²+2bx
x=1时有极大值3
∴3a+2b=0,a+b=3
∴a=-6,b=9
y’=-18x²+18x=0得x=0或x=1
y‘1或x