向量BC=AC-AB=(3,k-3)
当A是直角时,AB*AC=0,则-2+3k=0,k=2/3
当B是直角时,AB*BC=0,则-6+3(k-3)=0,k=5
当C是直角时,AC*BC=0,则3+k(k-3)=0,此时k无解