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to the angle
D, so that
BE be in the same straight line with AB, and produce FG 31. 1. to H; and through A drawb AH parallel to BG or EF, and join HB. Then, because the straight line HF falls upon the parallels AH, EF, the angles AHF, HFE, are together 29. 1. equal to two right angles; wherefore the angles BHF, HFE, are less than two right angles: But straight lines which with another straight line make the interior angles a 12 Ax. upon the same side less than two right angles, do meet if produced far enough: Therefore HB, FE shall meet if produced; let them meet in K, and through K draw KL, parallel to EA or FH, and produce HA, GB to the points L, M: Then HLKF is a parallelogram, of which the diameter is HK, and AG, ME are the parallelograms about HK; and LB, BF are the complements: therefore LB is 13. 1. equal to BF; But BF is equal to the triangle C; wherefore LB is equal to the triangle C; and because the angle 15.1. GBE is equalf to the angle ABM, and likewise to the angle D; the angle ABM is equal to the angle D: Therefore the parallelogram LB is applied to the straight line AB, is equal to the triangle C, and has the angle ABM equal to the angle D. Which was to be done.
PROP. XLV. PROB.
See N. To describe a parallelogram equal to a given rectilineal figure, and having an angle equal to a given rectilineal angle.
a 42. 1.
Let ABCD be the given rectilineal figure, and E the given rectilineal angle. It is required to describe a parallelogram equal to ABCD, and having an angle equal to E.
Join DB, and describe the parallelogram FH equal to the triangle ADB, and having the angle HKF equal to the
b 44. 1.
angle E; and to the straight line GH apply the parallelo- Boox I. gram GM equal to the triangle DBC, having the angle GHM equal to the angle E; and because the angle E is equal to each of the angles FKH, GHM, the angle FKH is equal to GHM: add to each of these the angle KHG; therefore the angles FKH, KHG, are equal to the
fore also KHG, GHM, are equal to two right angles; and because at the point H in the straight line GH, the two straight lines KH, HM upon the opposite sides of it make the adjacent angles equal to two right angles, KH is in the same straight lined with HM, and because the straight line 14. 1. HG meets the parallels KM, FG, the alternate angles MHG, HGF are equal: Add to each of these the angle HGL: Therefore the angles MHG, HGL, are equal to the angles HGF, HGL: But the angles MHG, HGL, are equale to two right angles; wherefore also the angles HGF, HGL are equal to two right angles, and FG is therefore in the same straight line with GL; and because KF is parallel to HG, and HG to ML; KF is parallel to ML; and KM, • 30. 1. FLare parallels; wherefore KFLM is a parallelogram; and because the triangle ABD is equal to the parallelogram HF, and the triangle DBC to the parallelogram GM; the whole rectilineal figure ABCD is equal to the whole parallelogram KFLM; therefore the parallelogram KFLM has been described equal to the given rectilineal figure ABCD, having the angle FKM equal to the given angle E. Which was to be done.'
COR. From this it is manifest how to a given straight line to apply a parallelogram, which shall have an angle equal to a given rectilineal angle, and shall be equal to a given rectilineal figure, viz. by applying to the given 44. 1. straight line a parallelogram equal to the first triangle ABD, and having an angle equal to the given angle.
a 11. 1.
PROP. XLVI. PROB.
To describe a square upon a given straight line.
Let AB be the given straight line; It is required to describe a square upon AB.
From the point A draw a AC at right angles to AB; and 3.1. makeb AD equal to AB, and through the point D draw DE 31. 1. parellel to AB, and through B draw BE parallel to AD; 34. 1. therefore ADEB is a parallelogram: whence AB is equal d to DE, and AD to BE: but BA is equal to AD; therefore the four straight lines BA, AD, DE, EB, are equal to one another, and the parallelogram ADEB is equilateral, like-D wise all its angles are right angles; because the straight line AD meeting the parallels AB, DE, the angles 29. 1. BAD, ADE are equal to two right angles: but BAD is a right angle; therefore also ADE is a right angle; A but the opposite sides of parallelo
grams are equal; therefore each of the opposite angles ABE, BED is a right angle; wherefore the figure ADEB is rectangular, and it has been demonstrated that it is equilateral; it is therefore a square, and it is described upon the given straight line AB: Which was to be done.
COR. Hence every parallelogram that has one right angle has all its angles right angles.
PROP. XLVII. THEOR.
IN any right-angled triangle, the square which is described upon the side subtending the right angle, is equal to the squares described upon the sides which contain the right angle.
Let ABC be a right-angled triangle having the right angle BAC; the square described upon the side BC is equal to the squares described upon BA, AC.
On BC describe the square BDEC, and on BA, AC the
b 31. 1.
• 30. def.
squares GB, HC; and through A drawb AL parallel to BD, Book I. or CE, and join AD, FC. Then, because each of the angles BAC, BAG is a right angles, the two straight lines AC, AG, upon the opposite sides of AB, make with it at the p point A the adjacent angles equal to two right angles; therefore CA is in the same straight lined with AG; for the same reason, AB and AH are in the same straight line; and because the angle DBC is equal to the angle FBA, each of them being a
right angle, add to each the
angle ABC, and the whole
angle DBA is equale to the whole FBC; and because the 2 Ax two sides AB, BD, are equal to the two FB, BC, each to each, and the angle DBA equal to the angle FBC; therefore the base AD is equal to the base FC, and the triangle 4. 1. ABD to the triangle FBC: Now the parallelogram BL is doubles of the triangle ABD, because they are upon the : 41. 1. same base BD, and between the same parallels BD, AL; and the square GB is double of the triangle FBC, because these also are upon the same base FB, and between the same parallels FB, GC. But the doubles of equals are equal to one another: Therefore the parallelogram BL is equal to the square GB: And, in the same manner, by joining AE, BK, tis demonstrated, that the parallelogram CL is equal to the square HC; Therefore the whole square BDEC is equal to the two squares GB, HC; and the square BDEC is described upon the straight line BC, and the squares GB, HC upon BA, AC: Wherefore the square upon the side BC is equal to the squares upon the sides BA, AC. There fore, in any right-angled triangle, &c. Q. E.D.
PROP. XLVIII. THEOR.
Ir the square described upon one of the sides of a triangle, be equal to the squares described upon the other two sides of it; the angle contained by these two sides is a right angle.
a 11. 1.
If the square described upon BC, one of the sides of the triangle ABC, be equal to the squares upon the other sides BA, AC, the angle BAC is a right angle.
From the point A draw a AD at right angles to AC, and make AD equal to BA, and join DC: Then, because DA is equal to AB, the square of DA is equal to the square of AB: To each of these add the square of AC; therefore the squares of DA, AC are equal to the squares of BA, AC: But the square of 47. 1. DC is equal to the squares of DA, AC, because DAC is a right angle; and the square of BC, by hypothesis, is equal to the squares of BA, AC; therefore the B square of DC is equal to the square of
BC; and therefore also the side DC is equal to the side BC. And because the side DA is equal to AB, and AC common to the two triangles DAC, BAC, the two DA, AC are equal to the two BA, AC; and the base DC is equal 8. 1. to the base BC; therefore the angle DAC is equal to the angle BAC; but DAC is a right angle; therefore also BAC is a right angle. Therefore, if the square, &c. Q. E. D.