Verified answer

(a + b)(a² - ab + b²) - (a - b)(a² + ab + b²) = (a³ + b³) - (a³ - b³) = a³ + b³ - a³ + b³ = 2b³

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(xz - yt)² + (yz + xt)² = (xz)² - 2xzyt + (yt)² + (yz)² + 2yzxt + (xt)² = x²z²  + y²t² + y²z²  + x²t² = z²(x² + y²) + t²(x² + y²) = (x² + y²)(z² + t²)