a/sinA=b/sinB B=2A
3/sinA=2√6/2sinAcosA
3=√6/cosA
cosA=√6/3已知数列an满足1/(1-a1) +......+1/(1-an)=n2+2n求数列的通项公式设bn=(1/1-an)1/(1-a1) +......+1/(1-an)=b1+b2+…bn=Sbn=n2+2nbn=Sbn-Sbn-1=2n+1n=1时,b1=S1=3符合bn=2n+1∴bn=2n+1bn=(1/1-an) an=1-(1/bn)=2n/2n+1)∴an=2n/2n+1)