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PROP. XXX. THEOR.
An exterior angle of a triangle is equal to both its opposite interior angles, and all the interior angles of a triangle are together, equal to two right angles.
The exterior angle BCD, formed by the production of the side AC of the triangle ABC, is equal to the two opposite interior angles CAB and CBA, and all the interior angles CAB, CBA and BCA of the triangles are together equal to two right angles.
For, through the point C, draw (I. 23.) the straight line CE parallel to AB. And, AB being parallel to CE, the interior angle BAC is (I.22.) equal to the exterior one ECD ; and, for the same reason, the alternate angle ABC is equal to BCE. Wherefore the two
angles CAB and ABC are equal to To JE DCE and ECB, or to the whole exterior angle BCD. Again, add the adjacent angle BCA & C L.
to the exterior angle BCD, and to the two interior angles CAB and ABC; and all the interior angles of the triangle ABC are together equal to the angles BCD and BCA on the same side of the straight line AD, that is, to two right angles. Cor. 1. Hence the two acute angles of a right angled triangle are together equal to one right angle; and hence each angle of an equilateral triangle is two-third parts of a right angle. Cor. 2. Hence if a triangle have its exterior angle, and one of its opposite interior angles, double of those in an
other triangle; its remaining opposite interior angle will also be double of the corresponding angle in the other. Schol. On the second corollary depends the construction of that invaluable reflecting angular instrument, called Hadley's quadrant or sextant. *
PROP. XXXI. THEOR. The interior angles of any rectilineal figure are together equal to twice as many right angles (abating four from the amount) as the figure has sides.
For assume a point O within the figure, and draw straight lines OA, OB, OC, OD, and OE, to the several corners. It is obvious, that the figure is thus resolved into as many triangles as it has sides, and whose collected angles must, by the last proposition, be equal to twice as many right angles. But the an- I. gles at the bases of these triangles A. constitute the internal angles of the figure. Consequently, from the whole amount, there is to be deducted the vertical angles about the point O, and which are (Def. 4.) equal to four right angles.
Cor. Hence all the angles of a quadrilateral figure are equal to four right angles, those of a pentelateral figure equal to six right angles, and so forth ; increasing the aggregate by two right angles, for each additional side.—The
same conclusion is derived from the successive application of triangles, by which the figure, at each accession, has the number of its sides increased by one, and the amount of its interior angles augmented by two right angles.
PROP. XXXII. THEOR.
The exterior angles of a rectilineal figure are together equal to four right angles.
The exterior angles DEF, CDG, BCH, ABI, and EAK of the rectilineal figure ABCDE are taken together
equal to four right angles.
For each exterior angle DEF, with its adjacent interior
one AED, is equal to two right angles. All the exterior angles, therefore, added to the interior angles, are equal to twice as many right angles as the figure has sides. Consequently the exterior angles are equal to the four right angles which, by the Proposition immediately preceding, were
abated, to form the aggregate of the interior angles. Cor. If the figure has a re-entrant angle BCD, the an
Schol. The amount of the exterior angles might be deduced from the successive deflections which a side would make before it has returned to its first position. Thus, in the first case, AF makes a complete circuit, changing into the positions EG, DH, CI, BK, and finally into AF again. But in the second case, AG, after making similar deflections, turns backwards at C from the position DK to CL.
PROP. XXXIII. THEOR.
If the opposite angles of a quadrilateral figure be equal, its opposite sides will be likewise equal and parallel.
In the quadrilateral figure ABCD, let the angle at B be equal to the opposite one at D, and the angle at A equal to that at C; the sides AB and BC are equal and parallel to DC and DA.
For all the angles of the figure being equal to four right angles (I. 31. cor.), and the opposite angles being mutually
equal, each pair of adjacent angles E - C must be equal to two right angles. - z Wherefore ABC and BCD are –" equal to two right angles, and the * 1)
lines AB and DC (I. 22. cor.) parallel; for the same reason, ABC and BAD being together equal to two right angles, the sides BC and AD, which limit them, are parallel. But (I. 26.) the parallel sides of the figure are also equal. Cor. Hence a quadrilateral figure contained by right angles has its opposite sides equal and parallel. +
PROP. XXXIV. PROB.
To draw a perpendicular from the extremity of a given straight line.
From the point B, to draw a perpendicular to AB, without producing that line. In AB take any point C, and on BC (I. 1. cor.) describe an isosceles triangle BDC, produce CD till DF be equal to it; and BF being joined, is the perpendicular required. . - -* * For, since by construction DF is equal to CD or BD, the triangle BDF is isosceles, and (I, 10.) the angle DBF equal to DFB; whence the angle CDB, - l being equal (I. 30.) to the interior angles / DBF and DFB, is double of DBF, or . . . 2. the angle DBF is half of CDB. But the triangle BDC being isosceles, the angle CBD is equal to BCD; consequently the angles DBF and DBC are the halves of the vertical and base angles of BDC, and therefore (I. 30.) the whole angle CBF is the half of two right angles, or it is equal to one right angle. Schol. This problem, of which the construction may be slightly modified, is often more convenient in practice than the one given in the corollary to Prop. 5. of this Book.
PROP. XXXV. PROB.
On a given finite straight line, to construct a square.
Let AB be the side of the square which it is required to COnstruct.