a-b+c=5,求a^2+b^2+c^2-2ab-2bc+2ca的值

a-b+c=5,求a^2+b^2+c^2-2ab-2bc+2ca的值

a^2+b^2+c^2-2ab-2bc+2ca
=(a-b)^2+2c(a-b)+c^2
=(a-b+c)^2
=25