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Trigonometric Formulas Identities

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TRIGONOMETRIC FORMULAS AND IDENTITIES

A. Reciprocal Relations
& &
sin 𝐴 = csc 𝐴 =
'(' ) (+, )

& &
cos 𝐴 = (.' ) sec 𝐴 = '0( )

& &
tan 𝐴 = '03 ) cot 𝐴 = 34, )

B. Pythagorean Relations

sin5 𝐴 + cos 5 𝐴 = 1

tan5 𝐴 + 1 = sec 5 𝐴

cot 5 𝐴 + 1 = csc 5 𝐴

C. By Definition

(+, ) '0( )
tan 𝐴 = '0( ) cot 𝐴 = (+, )

D. Functions of Sum and Difference of Angles

sin(𝐴 ± 𝐵) = sin 𝐴 cos 𝐵 ± cos 𝐴 sin 𝐵


cos(𝐴 ± 𝐵) = cos 𝐴 cos 𝐵 ∓ sin 𝐴 sin 𝐵
tan 𝐴 ± tan 𝐵
tan(𝐴 ± 𝐵) =
1 ∓ tan 𝐴 tan 𝐵
cot 𝐴 cot 𝐵 ∓ 1
cot(𝐴 ± 𝐵) =
cot 𝐵 ± cot 𝐴

E. Functions of Twice an Angle

sin 2𝐴 = 2 sin 𝐴 cos 𝐴

cos 2𝐴 = cos 5 𝐴 − sin5 𝐴

= 2 cos 5 𝐴 − 1

= 1 − 2 sin5 𝐴
2 tan 𝐴
tan 2𝐴 =
1 − tan5 𝐴
cot 5 𝐴 − 1
cot 2𝐴 =
2 cot 𝐴
F. Functions of Half-Angle

1 1 − cos 𝐴
sin 𝐴 = ±?
2 2

1 1 + cos 𝐴
cos 𝐴 = ±?
2 2

1 1 − cos 𝐴 sin 𝐴 1 − cos 𝐴


tan 𝐴 = ±? = =
2 1 + cos 𝐴 1 + cos 𝐴 sin 𝐴

1 1 + cos 𝐴 1 + cos 𝐴 sin 𝐴


cot 𝐴 = ±? = =
2 1 − cos 𝐴 sin 𝐴 1 − cos 𝐴

G. Sum and Difference of Functions


1 1
sin 𝐴 + sin 𝐵 = 2 sin (𝐴 + 𝐵) cos (𝐴 − 𝐵)
2 2
1 1
sin 𝐴 − sin 𝐵 = 2 cos (𝐴 + 𝐵) sin (𝐴 − 𝐵)
2 2
1 1
cos 𝐴 + cos 𝐵 = 2 cos (𝐴 + 𝐵) cos (𝐴 − 𝐵)
2 2
1 1
cos 𝐴 − cos 𝐵 = −2 sin (𝐴 + 𝐵) sin (𝐴 − 𝐵)
2 2
2 sin 𝐴 cos 𝐵 = sin(𝐴 + 𝐵) + sin(𝐴 − 𝐵)
2 cos 𝐴 sin 𝐵 = sin(𝐴 + 𝐵) − sin(𝐴 − 𝐵)
2 cos 𝐴 cos 𝐵 = cos(𝐴 + 𝐵) + cos(𝐴 − 𝐵)
2 sin 𝐴 sin 𝐵 = −cos(𝐴 + 𝐵) + cos(𝐴 − 𝐵)

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