¶ An Assumpt.
Forasmuch as in the eight booke in the 26. proposition it was proued, that like playne numbers haue that proportion the one to the other, that a square number hath to a square number: and likewise in the 24. of the same booke it was proued, that if two numbers haue that proportion the one to the other,* 1.1 that a square number hath to a square number, those numbers are like plaine numbers. Hereby it is manifest, that vnlike plaine numbers, that is, whose sides are not proportionall, haue not that proportion the one to the other, that a square number hath to a square number. For if they haue, then should they be like plaine numbers, which is contrary to the supposition. Wherfore vnlike plaine numbers haue not that propor∣tion the one to the other, that a square number hath to a square nūber. And therfore squares which haue that proportion the one to the other, that vnlike plaine numbers haue, shall haue their sides incommensurable in length (by the last part of the former proposition) for that those squares haue not that proportion the one to the other that a square number hath to a square number.