Smart, Edward Howard · London : Macmillian and Co. · leaf 223
n ratio (X andY will be on the same side of B if the ratio is positive; onopposite sides if negative). CROSS-RATIOS 195 Let AX, CY meet in V; then VD parallel to XBY will cut ABCat the point required. For pencils, take the range on any convenient transversal,and proceed as before. § 11. Piiop. The… … Fig. 101. The cross-ratios for the range A BCD may be expressed interms of ^, where 2^ is the angle at which these circlesintersect. We have at once from the figure, angle APB = compt. of angle BPC = angle CPD.Alsoangle APD + angle BPC = angle APC + angle BPD = 2 right angles... angle APD = suppt.…
Geometry, Projective
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