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(18) 2 cos=√√2+√2.

(19) 2 cos 11° 15'2+ √2+√2.

(20)

(21)

sin A. sin 2A + sin A. sin 44 + sin 24. sin 74
sin A. cos 24+ sin 24. cos 54+ sin A. cos 84
sin ✪ + sin (0 +$) + sin (8+24)

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=tan (8+4).

cos 8+cos(+$) + cos(8+2p)

(24)

(25) 2

(26) If tan a=

(22) 2 cos3 A-2 sin3 A= cos 24 (1+cos2 24).

(23) (3 sin A-4 sin3 A)2+(4 cos3 A - 3 cos A)2= 1.

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sin 2a. cos a
(1+ cos 2a) (1+ cos a)
cot (n-2)a.cot na +1
cot (n-2) a-cot na

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(27) Prove that tan

2

4 and cot

A

2

are the roots of the equation

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*168. The following examples are symmetrical, and each involve more than two angles :

Example 1. Prove that

sin(a+ẞ+7)=sin a. cos B. cos y + sin ß. cosy. cos a

+ sin y. cosa. cos ẞ- sin a. sin ẞ. sin y.

sin (a+B+y)=sin (a + B). cos y+cos (a + B) sin y

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And

2

β-γ

2

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sin B+sin y=2 sin+y.cos B-Y, [Art. 158]

.. sin a + sin ẞ+ sin y-sin (a+B+y)

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(1)

(2)

sin B.

cos (a+B+y)=cos a. cos B. cos y - cosa. sin ẞ. sin y
cos B. sin y. sina - cos y . sin a.
sin (a+ẞ-y)=sin a. cos B. cos y + sin ß. cosy. cos a
- sin y. cosa.cos ẞ+sin a sin ẞ sin y.

(3) cos (a-ẞ+y)=cosa.cos ẞ. cos y + cos a. sin ß. sin y

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(4) sin a+ sin ẞ-siny-sin (a+ẞ−y)

=4 sina.sin. sin+8.

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(6) sin 2a + sin 28+ sin 2y-sin 2 (a+B+y)

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(7) sin (8-7)+ sin (y-a) + sin (a-B)

+4 sin (2-7), sin (7a), sin (a)-0.

2

2

2

3)—0.

(8) sin (B+y-a)+sin (y+a−ẞ) + sin (a+ẞ − y)
-sin (a+B+y)=4 sin a. sin 3. sin y.

(9) sin (a+B+y) + sin (8+y-a) + sin (y+a-B)
− sin (a+ß − y)=4 cos a. cos ẞ. sin y.

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(11) cos 2x + cos 2y+cos 2z+ cos 2 (x + y + z)

2

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=4 cos (y + z). cos (z+x). cos (x+y). (12) cos (y + z−x) + cos (z+x − y) + cos(x+y-z) +cos(x+y+2)=4 cos x. cosy.cos z.

(13) cos2x+cos2 y + cos2 z + cos2 (x+y+z)

=2(1+cos (y + z). cos (z+x). cos(x+y)}.

(14) sin2 x + sin2 y + sin2 z+sin2 (x+y+z)

=2 {1 − cos (y + z). cos (z+x). cos (x+y)}.

(15) cos2x+cos2 y + cos2 z + cos2 (x + y − z)

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(16) cos a. sin (B− y) + cos ẞ. sin (y − a) + cos y . sin (a - ẞ)=0.

(17) sin a. sin (8 − y) + sin ß. sin (y − a) + sin y. sin (a — B) = 0.

(18) cos (a+B). cos (a− B) + sin (B+y) sin (B − y)

- cos (a+y). cos (a− y)=0.

(19) cos (-a). sin (8 - y) + cos (8-B). sin (y-a)

-cos (8-y). sin (B− a)=0.

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= cos 20+ cos 20+ cos 2x + 4 cos 0. cos. cos x + 1.

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2

CHAPTER XIII.**

ON ANGLES UNLIMITED IN MAGNITUDE. II.

169. THE words of the proofs (on pages 118, 119) of the 'A, B' formulæ apply to angles of any magnitude. The figures will be different for angles of different magnitude.

170. The figure for the 'A-B' formulæ on page 119 is the same for all cases in which A and B are each less than 90.

The figure given below is for the proof of the 'A + B' formulæ, when, A and B being each less than 90°, their sum is greater than 90°.

N

H

L

M

R

The words of the proof are precisely those of page 118.
We may notice however that

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171. Thus we have proved that the 'A, B' formulæ are true provided A and B each lie between 0° and 90o.

The student can prove them for any other values by drawing the proper figure.

The 'A, B' formulæ are therefore true for any values whatever of the angles A and B.

172. By the aid of the 'A, B' formulæ we can prove the formulæ of Art. 140.

Example. Prove that sin (90o +4)=cos A.

sin (90°+4)=sin 90° cos A+cos 90o sin A,

=1x cos 4+0 × sin A,

=cos A. Q. E. D.

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Draw the figures for the first four of the following examples.

(1) For the (A+B) formulæ, when A is greater than 90° and (A+B) less than 180o.

(2) For the (A–B) formulæ, when A and B each lie between 90° and 180o.

(3) For the (A+B) formulæ, when A lies between 90° and 180o, and (A + B) lies between 180o and 270o.

(4) For the (A - B) formulæ, when A lies between 180o and 270o, and (A – B) lies between 180o and A.

Deduce the six following formulæ from the 'A, B' formulæ.

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(11) Assuming that the formula sin (A+B)=sin A. cos B +cos A. sin B is true for all values of A and B, deduce the rest of the 'A, B' formulæ by the aid of the results on p. 107.

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