Text-book of Euclid's elements for the use of schools : books I - VI and XI (1900)
Hall, H. S. (Henry Sinclair), 1848-1934, Stevens, F. H. (Frederick Haller), 1853-1933 · MacMillan : London · leaf 431
if a straight lineis perpendicular to two straight lines which meet it in a plane, it isalso perpendicular to every straight line which meets it in that plane. A straight line drawn perpendicular to a plane is said to be anormal to that plane. 2. The foot of the perpendicular let fall from a… … Thus in the above figure the line ab is the projection of the lineAB on the plane PQ. Note. It will be proved hereafter (see page 446) that the pro-jection of a straight line on a plane is also a straight line. DEFINITIONS. 411 4. The inclination of a straight line to a plane is theacute angle…
Geometry · Mathematics
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