Davies, Charles, 1798-1876 · New York and Chicago, A. S. Barnes and company · leaf 52
two secants DE and EB inter-sect each other at E: then will the angleDEB be measured by half the interceptedarcs CA and DB. Draw the chord AF parallel to ED. D>Then, because AF and ED are parallel,and EB cuts them, the angles FAB andand DEB are equal (Bk. I. Th. xiv). But the angle FAB. at the… … THEOREM XVIII. An angle formed by two tangents is measured by half thedifference of the intercepted arcs. Let CD and DA be two tangents tothe circle at the points C and A : thenwill the angle CD A be measured by halfthe difference of the intercepted arcs CEAand CFA. For, draw the chord AF parallel…
Geometry · Plane trigonometry
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![An illustration from Nicolai Copernici Torinensis De reuolutionibus orbium coelestium, libri VI : Habes in hoc opere iam recens nato, & aedito, studiose lector, motus stellarum, tam fixarum, quàm erraticarum, cum ex ueteribus, tum etiam ex recentibus obseruationibus restitutos: & nouis insuper ac admirabilibus hypothesibus ornatos. Habes etiam tabulas expeditissimas, ex quibus eosdem ad quoduis tempus quàm facillime caculare poteris. Igitur eme, lege, fruere. [Line in Greek] (1543)](https://img.insideoutlibrary.com/img/200/ia-nicolaicopernici00cope-185-101.webp)
