Just follow this pattern to expand the square of a trinomial, (x+y+z)² = x² + y² + z² + 2xy +2xz +2yz, Solution:, (x+y+z)(x+y+z), So here are the calculations:, (x)(x)+(x)(y)+(x)(z)+(y)(x)+(y)(y)+(y)(z)+(z)(x)+(z)(y)+(z)(z), Therefore, we have:, x²+xy+xz+xy+y²+yz+xz+yz+z², Combine like terms:, x²+2xy+2xz+y²+2yz+z²