how much is the square of x+y+z
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(x+y+z)²
= (x+y+z) (x+y+z)
= x(x+y+z)+y(x+y+z)+z(x+y+z)
= x²+xy+xz+yx+y²+yz+zx+zy+z²
= x²+y²+z²+2xy+2yz+2zx
∴(x+y+z)²= x²+y²+z²+2xy+2yz+2zx
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