Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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For if it be poſſible, let it fall ſhort of it: and let R T be pro
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longed as farre as to A C in V: and then thorow V draw V X pa
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rallel to F D. Now, by the thing we have last demonſtrated, A X
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ſhall have the ſame proportion unto A R, as A F hath to A E.
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<
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>But A S hath alſo the ſame proportion to A R: Wherefore
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(a)
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A S
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is equall to A X, the part to the whole, which is impoſſi
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ble. </
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>The ſame abſurdity will follow if we ſuppoſe the Toint
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T to fall beyond the Line A C: It is therefore neceſſary that
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it do fall in the ſaid A C. </
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<
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>Which we propounded to be demonstrated.
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(a)
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By 9. of the
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fifth.
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<
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>LEMMA III.</
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>Let there be a Parabola, whoſe Diameter
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let be A B; and let the Right Lines A C and B D be ^{*} con
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tingent to it, A C in the Point C, and B D in B: And two
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Lines being drawn thorow C, the one C E, parallel unto
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the Diameter; the other C F, parallel to B D; take any
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Point in the Diameter, as G; and as F B is to B G, ſo let B
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G be to B H: and thorow G and H draw G K L, and H E
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M, parallel unto B D; and thorow M draw M N O parallel
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to
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A C,
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and cutting the Diameter in O: and the Line
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N P
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being drawn thorow
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N
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unto the Diameter let it be parallel
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to B D. </
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>I ſay that H O is double to G B.</
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* Or touch it.</
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For the Line M N O cutteth the Diameter either in G, or in other Points: and if it do
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cut it in G, one and the ſame Point ſhall be noted by the two letters G and O. </
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>Therfore F C,
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P N, and H E M being Parallels, and A C being Parallels to M N O, they ſhall make the
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Triangles A F C, O P N and O H M like to
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each other: Wherefore
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(a)
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O H ſhall be to
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H M, as A F to FC: and
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^{*} Permutando,
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O H ſhall be to A F, as H M to F C: But
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the Square H M is to the Square G L as the Line
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H B is to the Line B G, by 20. of our firſt Book
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of
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Conicks;
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and the Square G L is unto the
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Square F C, as the Line G B is to the Line B F:
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and the Lines H B, B G and B F are thereupon
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Proportionals: Therefore the
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(b)
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Squares
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H M, G L and F C and there Sides, ſhall alſo be
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Proportionals: And, therefore, as the (c)
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Square H M is to the Square G L, ſo is the Line
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H M to the Line F C: But as H M is to F C, ſo
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is O H to A F; and as the Square H M is to
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the Square G L, ſo is the Line H B to B G; that
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is, B G to B F: From whence it followeth that
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O H is to A F, as B G to B F: And
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Permu
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tando,
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O H is to B G, as A F to F B; But A F is double to F B: Therefore A B and B F
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are equall, by 35. of our firſt Book of
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Conicks:
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And therefore N O is double to G B:
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Which was to be demonſtrated.
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