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Let's say we change the volume of a gas under isothermal conditions, and we want to find the resulting pressure. Then, the equation of Boyle's law states that: p₂ = p₁ × V₁ / V₂ or p₂ / p₁ = V₁ / V₂. As we can see, the ratio of the final and initial pressure is the inverse of the ratio for volumes.