Trigonometric Identities - Miami

Trigonometric Identities

Sum and Difference Formulas

sin (x + y) = sin x cos y + cos x sin y

sin (x - y) = sin x cos y - cos x sin y

cos (x + y) = cos x cos y - sin x sin y

cos (x - y) = cos x cos y + sin x sin y

tan (x

+

y)

=

tan x+tan y 1-tan x tan y

tan (x - y)

=

tan x-tan y 1+tan x tan y

Half-Angle Formulas

sin

2

=

?

1-cos 2

tan

2

=

1-cos x sin x

cos

2

=

?

1+cos 2

tan

2

=

sin 1+cos

Double-Angle Formulas

tan

2

=

?

1-cos 1+cos

sin 2 = 2 sin cos cos 2 = 2 cos2 - 1

cos 2 = cos2 - sin2 cos 2 = 1 - 2 sin2

tan 2

=

2 tan 1-tan2

Product-to-Sum Formulas

sin x sin y

=

1 2

[cos (x - y)

-

cos (x

+

y)]

cos x cos y

=

1 2

[cos (x

-

y)

+

cos (x

+

y)]

sin x cos y

=

1 2

[sin (x +

y) +

sin (x

-

y)]

Sum-to-Product Formulas

sin x + sin y = 2 sin

x+y 2

cos

x-y 2

sin x - sin y = 2 sin

x-y 2

cos

x+y 2

cos x + cos y = 2 cos

x+y 2

cos

x-y 2

cos x - cos y = -2 sin

x+y 2

sin

x-y 2

The Law of Sines

sin A sin B sin C

=

=

a

b

c

Suppose you are given two sides, a, b and the angle A opposite the side A. The height of the triangle is h = b sin A. Then

1. If a < h, then a is too short to form a triangle, so there is no solution.

2. If a = h, then there is one triangle.

3. If a > h and a < b, then there are two distinct triangles.

4. If a b, then there is one triangle.

The Law of Cosines

a2 = b2 + c2 - 2bc cos A

b2 = a2 + c2 - 2ac cos B

c2 = a2 + b2 - 2ab cos C

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