給定AB = 27，BC = 32，AC = 29，那麽AB = 27是多少？

b+c=32......(2)

c+a=29......(3)

{(1)+(2)+(3)}÷2:

a+b+c = 44......(4)

(4)分別減去(2)、(3)和(1)得到:

a=44-32=12

b=44-29=15

c=44-27=17