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equivalent to twice the square of DB, and consequently the squares of AB and BC are equivalent to twice the squares of AD and DB. In every other case, whether the perpendicular BE fall

within or without the base AC, the , squares of AE, EC, the unequal segments of AC, are (II. 19. cor.) equivalent to twice the square of AD and twice the square of DE; add twice the square * P1: C of EB to both, and the squares of AE, EB and of CE, EB—or the squares of the hypotenuses AB, BC—are equivalent to twice the square of AD, and twice the squares of DE, EB, that is, (II. 10.) to twice the square of DB.

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A ID C E

PROP. XXIII. THEOR.

The square of the side of a triangle is greater or less than the squares of the base and the other side, according as the opposite angle is obtuse or acute, by twice the rectangle contained by the base and the distance intercepted between the vertex of that angle and the perpendicular.

In the oblique-angled triangle ABC, where the perpendicular BD falls without the base; the square of the side AB which subtends the oblique angle exceeds the squares of the sides AC and BC which contain it, by twice the rectangle under AC and CD.

For the square of AD, or of the sum of AC and CD, is (II. 15.) equivalent to the squares of these lines AC, CD,

together with twice their rectangle. Add the square of DB to each side, and the squares of AD, IB

DB, or (II. 10.) the square of AB is equivalent to the square of AC, and the 27 squares of CD, DB, together with twice

A. C ID

the rectangle AC, CD ; but the squares of CD, DB are (II. 10.) equivalent to the square of CB; whence the square of AB exceeds the squares of AC, BC, by twice the rectangle under AC and CD.

Again, in the acute angled triangle ABC, where the perpendicular BD falls within the triangle; the square of the side AB that subtends the 13 acute angle, is less than the squares of the containing sides AC, BC, by twice the rectangle under the base AC and its intercept- ATITC ed portion CD.

For the square of AD, or of the difference between AC and CD, is (II. 16.) equivalent to the squares of AC and CD, diminished by twice their rectangle. Add to each the square of DB, and the squares of AD and DB-or the square of AB--are equivalent to the square of AC, with the squares of CD and DB, or the square of BC, diminished by twice the rectangle under AC and CD. Consequently the square of AB is less than the squares of AC and BC, by twice the rectangle under AC and CD.

Cor. If the triangle ABC be isosceles, having equal sides AC and BC, the square of the base AB is equivalent to twice the rectangle under the side AC, and the adjacent segment AD made by the perpendicular BD, whether the vertical angle be obtuse or acute. For the square of AB is equivalent to the squares of AC and BC, or twice the square of AC increased or diminished by twice the rect-, angle under AC and CD ; that is, equivalent to twice the rectangle under AC and AD, the sum or difference of AC and CD.—This might also be demonstrated from the corollary to Prop. 10. Schol. When the three sides of a triangle are given, the segments of the base made by a perpendicular may be found either by Prop. 21. or Prop. 23., and thence the perpendicular can easily be determined from the application o Prop. 10. But half the rectangle under this perpendicular and the base will, by corollary to Prop. 5., express the area of the triangle.

PROP. XXIV. THEOR.

The squares of the sides of a rhomboid, are together equivalent to the squares of its diagonals.

Let ABCD be a rhomboid: The squares of all the sides AB, BC, CD, and AD, are together equivalent to the squares of the diagonals AC, BD. For the angles BCE and CBE are equal to the alternate angles DAE and ADE, and the B C

interjacent sides BC and AD are SU-7 equal; wherefore (I. 20.) the tri- /><7 angles BEC and DEA are equal.

- A- ID Consequently CE being equal to EA, the squares of AB, BC, are (II. 22.) equivalent to twice the square of AE and twice the square of BE; whence twice the squares of AB, BC, or the squares of all the sides of the rhomboid, are equivalent to four times the square of

AE and four times the square of BE, that is, to the squares of AC and BD.

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ELEMENTS

or

GEOMETRY.

BOOK III.

DEFINITIONS.

1. ANY portion of the circumference of a circle is called an arc, and the straight line 2– which joins the two extremities, a chord.

2. The space included between an arc and its chord, is named a segment.

3. A sector is the portion of a circle contained by two radii and the arc lying between them.

4. The tangent to a circle is a straight line which touches the circumference, or meets it only in a single point.

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