Cho 3 . ( a^2 + b^2 + c^2 = (a + b + c)^2. CMR a = b = c

bài 3:
a) Cho 3 . ( a^2 + b^2 + c^2 = (a + b + c)^2
CMR a=b=c
b) Cho (x + y + z) = 3 (xy + yz + zx)
CMR x=y=z