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chapter4.3r
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à 4.3ïProving Identities
äïWhich of the following equations is an identity?
â
è Prove that the equation, tan Θ ∙ cos Θï=ïsin Θ, is an identity.
êêêêë sin Θ
#è Proof:ë tan Θ ∙ cos Θï=ï───── ∙ cos Θï=ïsin Θ
êêêêë cos Θ
éSïIn Section 4.1, we proved that certain equations are identities
by using the original definitions of the trigonometric ratios.ïThat
could certainly still be done here as is shown in the following example.
êè tan Θ ∙ cos Θï=ï(y/x) ∙ (x/r)ï=ïy/rï=ïsin Θ
It is sometimes easier, however, to use the eight fundamental identities
and substitution to achieve the same results.ïIn the example above, we
could substitute the ratio identity, tan Θ = sin Θ/cos Θ, and then sim-
plify.êêêësin Θ
#êêtan Θ ∙ cos Θï=ï───── ∙ cos Θï=ïsin Θ
êêêêè cos Θ
This is considered to be a proof that the given equation is an identity.
To show an equation is not an identity, it is still necessary to find an
angle that makes a false sentence when the angle is substituted into the
equation.
è In Section 4.2, you were asked to simplify a given trigonometric ex-
pression into being another expression.ïThis is exactly the idea in
proving an identity.ïIn other words, you choose either side of the
equation and simplify it into being the other side.ïThe following ex-
ample shows the correct and incorrect form to write down a proof of an
identity.
êè CORRECTêêêêëINCORRECT !!!
a)ïProve tan Θ ∙ cos Θ = sin Θ.ë b)ïProve tan Θ ∙ cos Θ = sin Θ.
Proof:ëtan Θ ∙ cos ΘêêProof:è tan Θ ∙ cos Θ = sin Θ
êè sin Θêêêêè sin Θ
#ê=è───── ∙ cos Θêêêè───── ∙ cos Θ = sin Θ
êè cos Θêêêêè cos Θ
ê=èsin Θêêêêêïsin Θ = sin Θ
Thus, tan Θ ∙ cos Θ = sin Θ is
an identity.êêêêêè INCORRECT !!!
è The correct approach is to choose just one side and simplify it into
the other side.ïThe incorrect approach is to write down both sides
(thus assuming what you want to prove) and falsely concluding that you
have proven something is true when what you have done is assume that it
is true.
1ê Prove one of the following is an identity.
êêêA)ïcot Θ ∙ sin Θ = cos Θ
êêêB)ïsec Θ ∙ cot Θ = tan Θ
êêêC)ïå of ç
ü
a)ïProve cot Θ ∙ sin Θ = cos Θë b)ïProve sec Θ ∙ cot Θ = tan Θ is
is an identity.êêê not an identity.
Proof:ë cot Θ ∙ sin ΘêëProof:ïLet Θ = 45°.ïThen,
êëcos Θêêë sec 45° ∙ cot 45° = √2, but
#ê=è ───── ∙ sin Θêëtan 45° = 1.ïSince √2 ƒ 1,
êësin Θêêë sec Θ ∙ cot Θ = tan Θ is not
ê=è cos Θêêë an identity.
Thus, cot Θ ∙ sin Θ = cos Θ is
an identity.
Ç A
2ê Prove one of the following is an identity.
êêêA)ïsin Θ + cos Θ = 1/(sin Θ ∙ cos Θ)
êêêB)ïtan Θ + cot Θ = sec Θ ∙ csc Θ
êêêC)ïå of ç
ü
a)ïProve sin Θ + cos Θ =êëb)ïProve tan Θ + cot Θ =
1/(sin Θ ∙ cos Θ) is not anêèsec Θ ∙ csc Θ is an identity.
identity.
êêêêêïProof:è tan Θ + cot Θ
Proof:ïLet Θ = 30°.ïThen,êêë sin Θècos Θ
#sin 30° + cos 30° = (1 + √3)/2,êë =ï───── + ─────
but 1/(sin 30° ∙ cos 30°) =êêë cos Θèsin Θ
#4/√3.ïSince (1 + √3)/2 ƒ 4/√3,êë =ïsinìΘ + cosìΘ
#this is not an identity.êêêï─────────────
êêêêêêëcos Θ ∙ sin Θ
êêêêêêï=ï1/(cos Θ ∙ sin Θ)
êêêêêêï=ïsec Θ ∙ csc Θ
Ç B
3êProve one of the following is an identity.
êè 1 + sin Θêêêïsec Θ - cos Θ
#êA)ï─────────ï=ïcos ΘêïB)ï─────────────ï=ïsin Θ
êë tan Θêêêê tan Θ
êêêëC)ïå of ç
ü
a)ïProve (1 + sin Θ)/tan Θ =ê b)ïProve (sec Θ - cos Θ)/tan Θ =
cos Θ is not an identity.êësin Θ is an identity.
Proof:ïLet Θ = 30°.ïThen,êèProof:ïsec Θ - cos Θ
#(1 + sin 30°)/tan 30° = 3√3/2,êë=ï─────────────
but cos 30° = √3/2.ïSinceêêêïtan Θ
#3√3/2 ƒ √3/2, this is not anêê=ï(sec Θ - cos Θ) ∙ cot Θ
identity.êêêêë=ï(1/cos Θ - cos Θ)∙cot Θ
#êêêêêê=ï(1 - cosìΘ)/cos Θ∙cot Θ
#êêêêêê=ï(sinìΘ/cos Θ)∙(cos Θ/sin Θ)
êêêêêê=è sin Θ
Ç B
4êèProve one of the following is an identity.
#êïsinìx - 1êêêè sin x ∙ cot x
#ëA)ï─────────ï=ïcscìx - 1êB)ï─────────────ï=ïcos x
#êïcosìx - 1êêêè cos x ∙ csc x
êêêèC)ïå of ç
ü
#a)ïProve (sinìx - 1)/(cosìx - 1)è b)ïProve
#= cscìx - 1 is an identity.êè(sin x ∙ cot x)/(cos x ∙ csc x) =
êêêêêïcos x is not an identity.
#Proof: (sinìx - 1)/(cosìx -1)
#è =ï(-cosìx)/(-sinìx)êë Proof:ïLet x = 30°.ïThen, the
#è =ïcotìxêêêè left side is equal to 1/2, but
#è =ïcscìx - 1êêêthe right side is √3/2.ïSince
#êêêêêï1/2 ƒ √3/2, this is not an identi-
Thus, this is an identity.êè ty.
Ç A
5êèProve one of the following is an identity.
# A)ïsec x - sin x ∙ tan x = cos xêB)ï(1 + cotìx)∙tan x = sinìx
êêêë C)ïå of ç
üêëShow sec x - sin x ∙ tan x = cos x
êêè Proof:èsec x - sin x ∙ tan x
êêêè=ï1/cos x - sin x ∙ sin x/cos x
#êêêè=ï(1 - sinìx)/ cos x
#êêêè=ïcosìx/cos x
êêêè=ïcos x
Ç A
6êèProve one of the following is an identity.
#êïtan x ∙ cos xêêê 1 - secìx
# A)ï1 - ─────────────ï=ècos xêB)ï─────────ï=ïsecìx
#êëcsc xêêêë cosìx - 1
êêêëC)ïå of ç
#üêëShow (1 - secìx)/(cosìx - 1)ï=ïsecìx
#êêè Proof:è(1 - secìx)/(cosìx - 1)
#êêêè=ï(-tanìx)/(-sinìx)
#êêêè=ï(sinìx)/(cosìx) ∙ 1/sinìx
#êêêè=ï1/così x
#êêêè=ïsecì x
Ç B
7êèProve one of the following is an identity.
# A)ï(tanìx + 1)(cosìx - 1) = sinìxèB)ïsin x∙tan x = sec x - cos x
êêêëC)ïå of ç
üêëShow sin x∙tan x = sec x - cos x
êêè Proof:èsin x ∙ tan x
êêêè=ïsin x ∙ sin x/cos x
#êêêè=ïsinìx/cos x
#êêêè=ï(1 - cosìx)/cos x
êêêè=ï1/cos x - cos x
êêêè=ïsec x - cos x
Ç B
8êèProve one of the following is an identity.
A)ïsin x + cos x∙cot x = csc xë B)ïcsc x - cos x∙cot x = cos x
êêêëC)ïå of ç
üêëShow sin x + cos x∙cot x = csc x
êêè Proof:èsin x + cos x∙cot x
êêêè=ïsin x + cos x∙cos x/sin x
#êêêè=ï(sinìx + cosìx)/sin x
êêêè=ï1/sin x
êêêè=ïcsc x
Ç A
9êèProve one of the following is an identity.
# A)ïcsc x - cos x∙cot x = sin xë B)ï(tanìx + 1)(cosìx - 1) = secìx
êêêëC)ïå of ç
üêëShow csc x - cos x∙cot x = sin x
êêè Proof:ècsc x - cos x∙cot x
êêêè=ï1/sin x - cos x∙cos x/sin x
#êêêè=ï(1 - cosìx)/sin x
#êêêè=ïsinìx/sin x
êêêè=ïsin x
Ç A
10êïProve one of the following is an identity.
êtan x + 1
# A)ï─────────────ï=ïcot xêèB)ïsinÅx - cosÅxï=ï2∙sinìx - 1
ësec x + csc x
êêêëC)ïå of ç
#üêëShow sinÅx - cosÅxï=ï2∙sinìx - 1
#êêè Proof:èsinÅx - cosÅx
#êêêè=ï(sinìx + cosìx)(sinìx - cosìx)
#êêêè=ï(sinìx - cosìx)
#êêêè=ïsinìx - (1 - sinìx)
#êêêè=ï2∙sinìx - 1
Ç B