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- 148
- à 4.5ïDouble and Half-Angle Identities
-
- äïPlease use the Double and Half-Angle Identities to answer
- êêthe following questions.
- â
- êë Prove the identity, sin 2x ∙ sec xï=ï2∙sin x
-
-
- è Proof:ïsin 2x ∙ sec xï=ï2∙sin x ∙ cos x ∙ 1/cos xï=ï2∙sin x
- éSïIn the last section, we looked at the Addition Identities.
- While they will be helpful in some situations in later math courses,
- the Double and Half-Angle Identities will be needed frequently in
- differentiation and integration of trigonometric expressions in cal-
- culus and differential equations.ïThey are listed below.
- êêêèDouble-Angle Identities
- êêë 1)ïsin 2xï=ï2∙sin x ∙ cos x
-
- #êêë 2)ïcos 2xï=ïcosìx - sinìx
- #êêêêè =ï2∙cosìx - 1
- #êêêêè =ï1 - 2∙sinìx
-
- #êêë 3)ïtan 2xï=ï2∙tan x/(1 - tanìx)
-
- êêêè Half-Angle Identities
- #êêè ┌─────────────êêêï┌─────────────
- #4)ïcos x/2ï=ï± á(1 + cos x)/2ë5)ïsin x/2ï=ï± á(1 - cos x)/2
-
- ê 6)ïtan x/2ï=ï(1 -cos x)/sin xï=ïsin x/(1 + cos x)
-
- è Each of the double-angle identities can be proven by expressing 2x
- as x + x, and using the addition identities.ïFor example, sin 2xï=
- sin (x + x)ï=ïsin x ∙ cos x + cos x ∙ sin xï=ï2∙sin x ∙ cos x.ïIt
- is possible to prove the half-angle formulas by solving for the sin or
- #cos in the double-angle formulas.ïFor example, cos 2xï=ï1 - 2∙sinìx.
- #êêêè2∙sinìxï=ï1 - cos 2x
- #êêêèsinìxï=ï(1 - cos 2x)/2
- #êêêêê ┌──────────────
- #êêêèsin xï=ï± á(1 - cos 2x)/2
- è To prove the identity, sin 2x ∙ sec xï=ï2∙sin x, you can substi-
- tute the double-angle formula for sin 2x into the left side of the equa-
- tion.
- ë sin 2x ∙ sec xï=ï2∙sin x ∙ cos x ∙ 1/cos xï=ï2∙sin x
- 1ê Use a double-angle identity to evaluate
- #êêêê2∙così15° - 1
-
- êë A)ï1/2êêêêB)ï√3/2
-
- êë C)ï2êêêêïD)ïå of ç
- ü
- #êêêècos 2xï=ï2∙cosìx - 1
-
- #êêêècos 30°ï=ï2∙così15° - 1
-
- #êêêè√3/2ï=ï2∙così15° - 1
- Ç B
- 2ê Use a double-angle identity to evaluate
- êêêè2∙sin 22.5° ∙ cos 22.5°
-
- êë A)ï1êêêêïB)ï√2
-
- êë C)ï1/√2êêêë D)ïå of ç
- ü
- êêêèsin 2xï=ï2∙sin x ∙cos x
-
- êêêèsin 45°ï=ï2∙sin 22.5° ∙ cos 22.5°
-
- êêêè1/√2ï=ï2∙sin 22.5° ∙ cos 22.5°
- Ç C
- 3ê Use a double-angle identity to evaluate
- #êêêè2∙tan π/8/(1 - tanìπ/8)
-
- êë A)ï1êêêêïB)ï1/√2
-
- êë C)ï√3/2êêêë D)ïå of ç
- ü
- #êêêètan 2xï=ï2∙tan x/(1 - tanìx)
-
- #êêêètan π/4ï=ï2∙tan π/8/(1 - tanìπ/8)
-
- #êêêè1ï=ï2∙tan π/8/(1 - tanìπ/8)
- Ç A
- 4ê Use a half-angle identity to evaluate
- #êêêë ┌───────────────
- #êêêë á(1 - cos 60°)/2
- êë A)ï√3/2êêêë B)ï1/√2
-
- êë C)ï1/2êêêêD)ïå of ç
- #üêêêêï┌─────────────
- #êêêèsin x/2ï=ïá(1 - cos x)/2
- #êêêêê ┌───────────────
- #êêêèsin 30°ï=ïá(1 - cos 60°)/2
- #êêêêè ┌───────────────
- #êêêè1/2ï=ïá(1 - cos 60°)/2
- Ç C
- 5ê Use a half-angle identity to evaluate
-
- êêêè (1 - cos 120°)/sin 120°
- êë A)ï√3êêêê B)ï√3/2
-
- êë C)ï1/2êêêêD)ïå of ç
- ü
- êêêètan x/2ï=ï(1 - cos x)/sin x
-
- êêêètan 60°ï=ï(1 - cos 120°)/sin 120°
-
- êêêè√3ï=ï(1 - cos 120°)/sin 120°
- Ç A
- 6ë Prove one of the following is an identity.
-
- #ëcosìx - sinìxêêêë sinì2x
- # A)ï─────────────ï=ï2∙cot 2xê B)ï──────────ï=ï2∙sinìx
- ësin x ∙ cos xêêêè 1 - cos 2x
-
- êêêè C)ïå of ç
- ü
- #êêêê cosìx - sinìx
- #êêêê ─────────────
- êêêê sin x ∙ cos x
-
- êêêêè cos 2x
- #êêêè=è ────────────
- êêêê 1/2 ∙ sin 2x
-
- êêêè=è 2∙cot 2x
- Ç A
- 7ë Prove one of the following is an identity.
-
- êêêêêêë2
- # A)ïcosÅx - sinÅxï=ïsin 2xêèB)ï──────────ï=ïsecìx
- êêêêêê 1 + cos 2x
-
- êêêè C)ïå of ç
- ü
- êêêêë 2
- #êêêê ───────────
- êêêê 1 +ïcos 2x
-
- êêêêê2
- #êêêè=è ──────────────
- #êêêê 1 + 1 - 2∙sinìx
-
- êêêêë 2
- #êêêè=è ─────────────
- #êêêê 2(1 - sinìx)
- #êêêè=è 1/cosìx
- #êêêè=è secìx
- Ç B
-
-