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à 4.1ïThe Fundamental Trigonometric Identities
äïPlease show which of the following equations is an iden-
êêtity.
â
êïShow that the equation, sin Θ = 1/csc Θ, is an identity.
êë sin Θï=ïy/rï=ï1/(r/y)ï=ï1/csc Θ
êë Thus, sin Θ = 1/csc Θ is an identity.
êêê (Please see Details)
éSïA trigonometric identity is an equation, involving trigonometric
functions, that is true for every value of the variable that is in the
domains of the particular trigonometric functions in the equation.ïFor
#example, since sinìΘ + cosìΘ = 1 is a true sentence for all values of Θ,
it is an identity.ïThere are eight fundamental trigonometric identi-
ties.ïThey are listed below by category.
êïReciprocal IdentitiesêèPythagorean Identities
#êï1.ïsin Θ = 1/csc Θêë4.ïsinìΘ + cosìΘ = 1
#êï2.ïcos Θ = 1/sec Θêë5.ï1 + tanìΘ = secìΘ
#êï3.ïtan Θ = 1/cot Θêë6.ï1 + cotìΘ = cscìΘ
êêêëRatio Identities
êêêï7.ïtan Θ = sin Θ/cos Θ
êêêï8.ïcot Θ = cos Θ/sin Θ
è The first method we will use to prove an equation is an identity is
by using the original definitions of the trigonometric ratios. They are
given below.
êêêè 1.ïsin Θ = y/rêï4.ïcsc Θ = r/y
êêêè 2.ïcos Θ = x/rêï5.ïsec Θ = r/x
êêêè 3.ïtan Θ = y/xêï6.ïcot Θ = x/y
@fig4101.bmp,10,160
êêêê In the example, to show that sin Θ =
êêêè 1/csc Θ is an identity, we can start with one
êêêè side of the equation and substitute in the cor-
êêêè rect ratio.
êêêêêë sin Θ = y/r
êêêè We can then algebraically manipulate this ratio
êêêè until it equals the other side of the equation.
êêêêsin Θï=ïy/rï=ï1/(r/y)ï=ï1/csc Θ
êêêè At this point we have established that the
êêêè equation, sin Θ = 1/csc Θ, is an identity.
êêêê To show an equation is not an identity,
êêêè you should substitute a value of the variable
êêêè that produces a false sentence.ïThen, the
êêêè equation can not be an identity.ïFor example,
êêêè to show that sin Θ = 1 - cos Θ is not an
identity, we could substitute 30° for Θ.ïThen sin 30° = 1/2, and
#1 - cos 30° = 1 - √3/2 ƒ 1/2.ïTherefore, sin Θ = 1 - cos Θ is not an
identity.ïThis technique of showing that an equation is not an identi-
ty is called proof by counterexample.ïThus, in order to show that an
equation is not true for all angles, you just have to find one angle
where it is false.
1ïCan you show which of the following equations is an identity?
êêè1êêêè1
#èA)ïcos Θ = ─────ê B)ïtan Θ = ─────ë C)ïå of ç
êê csc Θêêë cot Θ
ü
a)ïShow that cos Θ = 1/csc Θë b)ïShow that tan Θ = 1/cot Θ is an
è is not an identity.êêïidentity.
è Let Θ = 30°.ïThen, cos 30°êêë tan Θ
è = √3/2, but 1/csc 30° = 2.êêè =ïy/x
#è Since √3/2 ƒ 2, the equa-êêë=ï1/(x/y)
è tion, cos Θ = 1/csc Θ, isêêë=ï1/cot Θ
è not an identity.êêë Therefore, tan Θ = 1/cot Θ is an
êêêêêëidentity.
Ç B
2ïCan you show which of the following equations is an identity?
#èA)ï1 + tanìΘ = secìΘè B)ïcscìΘ = 1 - secìΘè C)ïå of ç
ü
#a)ïShow that 1 + tanìΘ = secìΘè b)ïShow that cscìΘ = 1 - secìΘ is
è is an identity.êêïnot an identity.
#êè1 + tanìΘêêïLet Θ = 30°.ïThen, cscì30° = 4, but
#ê=ï1 + (y/x)ìêê 1 - secì30° = 1 - (2/√3)ì = -1/3.
#ê=ï1 + yì/xìêêïSince 4 ƒ -1/3, cscìΘ = 1 - secìΘ is
#ê=ï(xì + yì)/xìêë not an identity.
#ê=ïrì/xì
#ê=ï(r/x)ì
#ê=ïsecìΘ
#Therefore, 1 + tanìΘ = secìΘ
is an identity.
Ç A
3ïCan you show which of the following equations is an identity?
êêè1
#èA)ïsec Θ = ─────ë B)ïcotìΘ + tanìΘ = 1ëC)ïå of ç
êê cos Θ
ü
#a)ïShow that sec Θ = 1/cos Θë b)ïShow that cotìΘ + tanìΘ = 1 is
è is an identity.êêïnot an identity.
#êèsec Θêêë Let Θ = 45°.ïThen, cotì45° + tanì45°
#ê=ïr/xêêê = 1 + 1 = 2.ïSince 2 ƒ 1, cotìΘ +
#ê=ï1/(x/r)êêè tanìΘ = 1 is not an identity.
ê=ï1/ cos Θ
Therefore, sec Θ = 1/cos Θ
is an identity.
Ç A
4ïCan you show which of the following equations is an identity?
êêè1
#èA)ïsec Θ = ─────ë B)ï1 + cotìΘ = secìΘè C)ïå of ç
êê sin Θ
ü
#a)ïShow that cos Θ = 1/csc Θë b)ïShow that 1 + cotìΘ = secìΘ is an
è is not an identity.êêïidentity.
#è Let Θ = 30°.ïThen, cos 30°êêë 1 + cotìΘ
#è = √3/2, but 1/csc 30° = 2.êêè =ï1 + (x/y)ì
#è Since √3/2 ƒ 2, the equa-êêë=ï1 + xì/yì
#è tion, cos Θ = 1/csc Θ, isêêë=ï(yì + xì)/yì
#è not an identity.êêêê=ïrì/yì
#êêêêêêë =ï(r/y)ì
#êêêêêêë =ïsecìΘ
#êêêêêè Therefore, 1 + cotìΘ = secìΘ is
êêêêêè an identity.
Ç B
5ïCan you show which of the following equations is an identity?
êêè1
#èA)ïsin Θ = ─────ë B)ïsecìΘ = 1 - cotìΘè C)ïå of ç
êê cos Θ
ü
#a)ïShow that sin Θ = 1/cos Θë b)ïShow that secìΘ = 1 - cotìΘ is
is not an identity.êêïnot an identity.
#Let Θ = 30°.ïThen, sin 30°ê Let Θ = 45°.ïThen, secì45° = 2, but
#= 1/2, but 1/cos 30° = 2/√3.ê1 - cotì45° = 0.ïSince 2 ƒ 0, secìΘ
#Since 1/2 ƒ 2/√3, sin Θ =êè= 1 - cotìΘ is not an identity.
1/ cos Θ is not an identi-
ty.
Ç C
6ïCan you show which of the following equations is an identity?
êêè1
#èA)ïcsc Θ = ─────ë B)ïtan Θ - 1 = sec Θè C)ïå of ç
êê sin Θ
ü
a)ïShow that csc Θ = 1/sin Θë b)ïShow that tan Θ - 1 = sec Θ is
è is an identity.êêïnot an identity.
êècsc Θêêë Let Θ = 45°.ïThen, tan 45° - 1 = 0,
#ê=ïr/yêêê but sec 45° = √2.ïSince 0 ƒ √2,
ê=ï1/(y/r)êêè tan Θ - 1 = sec Θ is not an identity.
ê=ï1/ sin Θ
Therefore, csc Θ = 1/sin Θ
is an identity.
Ç A
7ïCan you show which of the following equations is an identity?
êêè1
#èA)ïcot Θ = ─────ë B)ïsinìΘ + cosìΘ = 1ëC)ïå of ç
êê csc Θ
ü
#a)ïShow that cot Θ = 1/csc Θë b)ïShow that sinìΘ + cosìΘ = 1 is an
è is not an identity.êêïidentity.
#è Let Θ = 30°.ïThen, cot 30°êêë sinìΘ + cosìΘ
#è = √3/1, but 1/csc 30° = 2.êêè =ï(y/r)ì + (x/r)ì
#è Since √3/1 ƒ 2, the equa-êêë=ïyì/rì + xì/rì
#è tion, cot Θ = 1/csc Θ, isêêë=ï(yì + xì)/rì
#è not an identity.êêêê=ïrì/rì
êêêêêêë =ï1
#êêêêêè Therefore, sinìΘ + cosìΘ = 1 is
êêêêêè an identity.
Ç B
8ïCan you show which of the following equations is an identity?
êê cos Θêêë sin Θ
#èA)ïsin Θ = ─────ê B)ïtan Θ = ─────ë C)ïå of ç
êê sec Θêêë cos Θ
ü
a)ïShow that sin Θ = cosΘ/secΘè b)ïShow that tan Θ = sin Θ/cos Θ is
è is not an identity.êêïan identity.
è Let Θ = 30°.ïThen, sin 30°êêë tan Θ
è = 1/2, but cos 30°/sec 30° =êêï=ïy/x
#è 3/4.ïSince 1/2 ƒ 3/4, the equa-êë=ï(y/r)/(x/r)
è tion, sin Θ = cos Θ/sec Θ isêêï=ïsin Θ/cos Θ
è not an identity.
êêêêêè Therefore, tan Θ = sin Θ/cos Θ is
êêêêêè an identity.
Ç B