(a+b+c)^2=[(a+b)+c]^2=(a+b)^2+2(a+b)c+c^2=a^2+b^2+c^2+2ab+2bc+2ac
[(a+b)+c]^2=(a+b)^2+2(a+b)c+c^2=a^2+2ab+b^2+2ac+2bc+c^2=a^2+b^2+c^2+2ab+2ac+2bc
题目好像有问题