Triangle Definition of Trig Functions
In the right triangle at the right, the angle at the base has been
denoted by θ, the opposite side
is the altitude a, the adjacent side
is the base b and the hypotenuse is
the diagonal side c. In terms of these the six trig functions
SineCosineTangentCotangentSecantCosecant
are defined to be:
sinθtanθsecθ=HypOpp=ca=AdjOpp=ba=AdjHyp=bccosθcotθcscθ=HypAdj=cb=OppAdj=ab=OppHyp=ac
Memorize this!
There is a Mnemonic to remember the first three of these:
Mnemonic:SOH CAH TOA
which stands for
Sin=Opp/Hyp Cos=Adj/Hyp Tan=Opp/Adj
Based on these we have the standard quotient and reciprocal identities:
Quotient and Reciprocal Identities tanθsecθ=cosθsinθ=cosθ1cotθcscθ=sinθcosθ=tanθ1=sinθ1
Given the right triangle shown with adjacent side 5 and opposite side 12,
find the hypotenuse and the six trig functions for the angle θ.
By the Pythagorean Theorem, the hypotenuse is
Hyp=52+122=169=13.
So, the six trig functions are:
sinθtanθsecθ=HypOpp=1312=AdjOpp=512=AdjHyp=513cosθcotθcscθ=HypAdj=135=OppAdj=125=OppHyp=1213
Given the right triangle shown with adjacent side 0.9 and opposite side 1.2,
find the hypotenuse and the six trig functions for the angle θ.
By the Pythagorean Theorem, the hypotenuse is
Hyp=.92+1.22=.81+1.44=2.25=1.5.
So, the six trig functions are:
sinθtanθsecθ=HypOpp=1.51.2=54=AdjOpp=0.91.2=34=AdjHyp=0.91.5=35cosθcotθcscθ=HypAdj=1.50.9=53=OppAdj=1.20.9=43=OppHyp=1.21.5=45
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