4b²+3c²=4根号下3bc(2b)²-2×2根号下3bc+(根号下3c)²=0(2b-根号下3c)²=02b=根号下3cb/c=根号下3/2b/sinb=c/sinc b/c=sinb/sinc=sinb/1=cosa=根号下3/2 所以 cosa=根号下3/2...