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PROPOSITION XII. THEOREM.

In obtuse-angled triangles, if a perpendicular be drawn from either of the acute angles to the opposite side produced, the square on the side subtending the obtuse angle is greater than the squares on the sides containing the obtuse angle, by twice the rectangle contained by the side, upon which, when produced, the perpendicular falls, and the straight line intercepted without the triangle between the perpendicular and the obtuse angle.

Let ABC be an obtuse-angled ▲, having ▲ ACB obtuse.
From A draw AD 1 to BC produced.

Then must sq. on AB be greater than sum of sqq. on BC, CA by twice rect. BC, CD.

For şince BD is divided into two parts in C,

sq. on BD=sum of sqq. on BC, CD, and twice rect. BC, CD.

II. 4.

Add to each sq. on DA: then sum of sqq. on BD, DA=sum of sqq. on BC, CD, DA and twice rect. BC, CD.

Now sqq. on BD, DA=sq. on AB,
and sqq. on CD, DA=sq. on CA ;

I. 47.

I. 47.

.. sq. on AB=sum of sqq. on BC, CA and twice rect. BC, CD. .. sq. on AB is greater than sum of sqq. on BC, CA by twice rect. BC, CD.

Q. E. D.

Ex. The squares on the diagonals of a trapezium are together equal to the squares on its two sides, which are not parallel, and twice the rectangle contained by the sides, which are parallel.

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PROPOSITION XIII.

THEOREM.

In every triangle, the square on the side subtending any of the acute angles is less than the squares on the sides containing that angle, by twice the rectangle contained by either of these sides and the straight line intercepted between the perpendicular, let fall upon it from the opposite angle, and the acute angle.

FIG. 1.

FIG. 2.

B

с

Let ABC be any ▲, having the

ABC acute.

From A draw AD 1 to BC or BC produced.

B

A

Then must sq. on AC be less than the sum of sqq. on AB, BC, by twice rect. BC, BD.

For in Fig. 1 BC is divided into two parts in D,

and in Fig. 2 BD is divided into two parts in C; .. in both cases

sum of sqq. on BC, BD=sum of twice rect. BC, BD and sq. on CD. II. 7.

Add to each the sq. on DA, then

sum of sqq. on BC, BD, DA=sum of twice rect. BC, BD and sqq. on CD, DA ;

.. sum of sqq. on BC, AB=sum of twice rect. BC, BD and sq. on AC; I. 47.

.. sq. on AC is less than sum of sqq. on AB, BC by twice rect. BC, BD.

The case, in which the perpendicular AD coincides with AC, needs no proof.

Q. E. D.

Ex. Prove that the sum of the squares on any two sides of a triangle is equal to twice the sum of the squares on half the base and on the line joining the vertical angle with the middle point of the base,

A

PROPOSITION XIV.

PROBLEM.

To describe a square that shall be equal to a given rectilinear

figure.

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Let A be the given rectil. figure.
It is reqd. to describe a square that shall=A.
Describe the rectangular □ BCDE=A.
Then if BE=ED the □ BCDE is a square,
and what was reqd. is done.

Take from each the square on GE.

E

D

But if BE be not=ED, produce BE to F, so that EF=ED. Bisect BF in G; and with centre G and distance GB, describe the semicircle BHF.

Produce DE to H and join GH.

Then rect. BE, EF=sq. on EH.
But rect. BE, EF=BD, :: EF=ED;

.. sq. on EH=BD;

.. sq. on EH=rectil. figure A.

Then, ·.· BF is divided equally in G and unequally in E, .. rect. BE, EF together with sq. on GE

II. 5.

=sq. on GF =sq. on GH

=sum of sqq. on EH, GE.

I. 45.

I. 47.

Q. E. F.

Ex. Shew how to describe a rectangle equal to a given square, and having one of its sides equal to a given straight line.

Miscellaneous Exercises on Book II.

1. In a triangle, whose vertical angle is a right angle, a straight line is drawn from the vertex perpendicular to the base; shew that the square on either of the sides adjacent to the right angle is equal to the rectangle contained by the base and the segment of it adjacent to that side.

2. The squares on the diagonals of a parallelogram are together equal to the squares on the four sides.

3. If ABCD be any rectangle, and O any point either within or without the rectangle, show that the sum of the squares on OA, OC is equal to the sum of the squares on OB, OD.

4. If either diagonal of a parallelogram be equal to one of the sides about the opposite angle of the figure, the square on it shall be less than the square on the other diameter, by twice the square on the other side about that opposite angle.

5. Produce a given straight line AB to C, so that the rectangle, contained by the sum and difference of AB and AC, may be equal to a given square.

6. Shew that the sum of the squares on the diagonals of any quadrilateral is less than the sum of the squares on the four sides, by four times the square on the line joining the middle points of the diagonals.

7. If the square on one perpendicular from the vertex of a triangle is equal to the rectangle, contained by the segments of the base, the vertical angle is a right angle.

8. Produce a given straight line so that the rectangle contained by the whole line thus produced and another given straight line may be equal to the square on the produced part.

9. ABC is a triangle right-angled at A; in the hypote nuse two points D, E are taken such that BD=BA and CE=CA; shew that the square on DE is equal to twice the rectangle contained by BE, CD.

10. In any quadrilateral the squares on the diagonals are together equal to twice the sum of the squares on the straight lines joining the middle points of opposite sides.

11. If straight lines be drawn from each angle of a triangle to bisect the opposite sides, four times the sum of the squares on these lines is equal to three times the sum of the squares on the sides of the triangle.

12. CD is drawn perpendicular to AB, a side of the triangle ABC, in which AC-AB. Shew that the square on CD is equal to the square on BD together with twice the rectangle AD, DB.

13. If in any triangle BAC a line AD be drawn bisecting BC in D, shew that the sum of the squares on AB, AC is equal to twice the sum of the squares on AD, BD.

14. If ABC be an equilateral triangle, and AD, BE be perpendiculars to the opposite sides intersecting in F; shew that the square on AB is equal to three times the square on AF.

15. Divide a given straight line into two parts, so that the rectangle contained by them shall be equal to the square described upon a straight line, which is less than half the line divided.

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