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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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126 Chapter 3 The trigonometric functions

Table3.1

Common trigonometricidentities.

tanA= sinA

cosA

sin(A ±B) =sinAcosB ±sinBcosA

cos(A ±B) =cosAcosB ∓sinAsinB

tan(A ±B) = tanA±tanB

1∓tanAtanB

2sinAcosB =sin(A +B) +sin(A −B)

2cosAcosB =cos(A +B) +cos(A −B)

2sinAsinB =cos(A −B) −cos(A +B)

sin 2 A+cos 2 A=1

1 +cot 2 A = cosec 2 A

tan 2 A+1=sec 2 A

cos2A=1−2sin 2 A=2cos 2 A−1=cos 2 A−sin 2 A

sin2A=2sinAcosA

sin 2 A = 1−cos2A

2

cos 2 A = 1+cos2A

2

Note: sin 2 Ais the notationused for (sinA) 2 .Similarly cos 2 A means (cosA) 2 .

Example3.4 Show that

tanA+cotA

maybewritten as

2

sin2A

Solution We have

and so

tanA= sinA

cosA ,

cotA=cosA sinA

tanA+cotA= sinA

cosA + cosA

sinA

= sin2 A +cos 2 A

sinAcosA

1

= usingthe identity sin 2 A +cos 2 A = 1

sinAcosA

2

=

2sinAcosA

= 2 using the identity sin2A = 2sinAcosA

sin2A

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