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chapter4.4r
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à 4.4ïThe Addition Formulas
äïPlease use the Addition Formulas to answer the following
êêquestions.
â
êë Change cos (45° + φ) to functions of φ only.
êècos (45° + φ)ï=ïcos 45° ∙ cos φ - sin 45° ∙ sin φ
êêêè =ï(cos φ - sin φ)/√2
éSïIn addition to the eight fundamental identities and the many
identities that are provable using the eight fundamental identities,
there are some other very important identities that will be especially
helpful in later math courses.ïThey are the addition formulas, the
double-angle and half-angle identities, and the product and sum identi-
ties.ïThese will be covered in the next three sections.ïIn this sec-
tion we will focus on the addition formulas.ïThey are listed below.
êë 1)ïsin (A + B)ï=ïsin A∙cos B + cos A∙sin B
êë 2)ïsin (A - B)ï=ïsin A∙cos B - cos A∙sin B
êë 3)ïcos (A + B)ï=ïcos A∙cos B - sin A∙sin B
êë 4)ïcos (A - B)ï=ïcos A∙cos B + sin A∙sin B
êë 5)ïtan (A + B)ï=ï(tan A + tan B)/(1 - tan A∙tan B)
êë 6)ïtan (A - B)ï=ï(tan A - tan B)/(1 + tan A∙tan B)
It is possible to locate angles A, B, and A - B on the unit circle,
label the coordinates of the points, and use the distance formula on
the resulting chords to derive the formula, cos (A - B)ï=
cos A∙cos B + sin A∙sin B.ïThe formula for cos (A + B) can be found by
expressing it as cos(A -(-B)) and simplifying the first formula.ïThe
remaining identities can also be derived from ç two identities.
These six identities are used to simplify trigonometric functions of the
sum or difference of two angles.
è In the example, cos (45° + φ) can be changed by using the cos of the
sum of two angles identity.
êïcos (45° + φ)ï=ïcos 45° ∙ cos φ - sin 45° ∙ sin φ
êêêè=ï1/√2 ∙ cos φ - 1/√2 ∙ sin φ
êêêè=ï(cos φ - sin φ)/√2
1êê Simplifyïsin (φ - 60°)
êè A)ïsin φ - cos φêê B)ïsin φ - 2∙cos φ
êè C)ï(sin φ - √3∙cos φ)/2ê D)ïå of ç
ü
ê sin (φ - 60°)ï=ïsin φ∙cos 60° - cos φ∙sin 60°
êêêï=ï1/2 ∙ sin φ - √3/2 ∙ cos φ
êêêï=ï(sin φ - √3 ∙ cos φ)/2
Ç C
2êê Simplifyïcos (π/6 - φ)
ê A)ï√3 ∙ sin φ - 2 ∙ cos φë B)ï(√3 ∙ cos φ + sin φ)/2
ê C)ï2 ∙ cos φ - sin φêè D)ïå of ç
ü
ê cos (π/6 - φ)ï=ïcos π/6∙cos φ + sin π/6∙sin φ
êêêï=ï√3/2 ∙ cos φ + 1/2 ∙ sin φ
êêêï=ï(√3 ∙ cos φ + sin φ)/2
Ç B
3êê Simplifyïtan (45° + φ)
êè A)ïtan φ/(1 - tan φ)êè B)ï(1 + tan φ)/(1 - tan φ)
êè C)ï(1 - tan φ)/tan φêè D)ïå of ç
ü
ê tan (45° + φ)ï=ï(tan 45° + tan φ)/(1 - tan 45° ∙ tan φ)
êêêï=ï(1 + tan φ)/(1 - tan φ)
Ç B
4êè Use your calculator to evaluate
êêïsin 142° ∙ cos 36° - cos 142° ∙ sin 36°
êë A)ï.9613êêêB)ï.7613
êë C)ï.8324êêêD)ïå of ç
ü
êêïsin 142° ∙ cos 36° - cos 142° ∙ sin 36°
êêêè=ïsin (142° - 36°)
êêêêï≈ï.9613
Ç A
5êè Use your calculator to evaluate
êêïcos 42° ∙ cos 58° - sin 42° ∙ sin 58°
êë A)ï.2134êêêB)ï-.3145
êë C)ï-.1736êêë D)ïå of ç
ü
êêïcos 42° ∙ cos 58° - sin 42° ∙ sin 58°
êêêè=ïcos (42° + 58°)
êêêêï≈ï-.1736
Ç C
6êè Use your calculator to evaluate
êê (tan 32° + tan 18°)/(1 - tan 32° ∙ tan 18°)
êë A)ï1.1918êêë B)ï1.9785
êë C)ï2.3166êêë D)ïå of ç
ü
êê (tan 32° + tan 18°)/(1 - tan 32° ∙ tan 18°)
êêêè=ïtan (32° + 18°)
êêêêï≈ï1.1918
Ç A
7ïProve one of the following equations is an identity.
ë A)ïsin (3π/2 + φ)ï=ï-cos φëB)ïcos (3π/2 - φ)ï=ïcos φ
êêêë C)ïå of ç
ü
êêëProveïsin (3π/2 + φ)ï=ï-cos φ
êêëProof:ïsin (3π/2 + φ)
êêêï=ïsin 3π/2 ∙ cos φ + cos 3π/2 ∙sin φ
êêêï=ï-1 ∙ cos φ + 0 ∙sin φ
êêêï= -cos φ
Ç A
8ïProve one of the following equations is an identity.
ë A)ïcos (π + φ)ï=ï-sin φëB)ïtan (π + φ)ï=ïtan φ
êêêë C)ïå of ç
ü
êêëProveïtan (π + φ)ï=ïtan φ
êêëProof:ïtan (π + φ)
êêêï=ï(tan π + tan φ)/(1 - tan π ∙tan φ)
êêêï=ï(0 + tan φ)/(1 - 0 ∙tan φ)
êêêï=ïtan φ
Ç B
9ïProve one of the following equations is an identity.
ë A)ïcos (π - φ)ï=ï-cos φëB)ïsin (π + φ)ï=ïcos φ
êêêë C)ïå of ç
ü
êêëProveïcos (π - φ)ï=ï-cos φ
êêëProof:ïcos (π - φ)
êêêï=ïcos π ∙ cos φ + sin π ∙ sin φ
êêêï=ï-1 ∙ cos φ + 0 ∙ sin φ
êêêï=ï-cos φ
Ç A