[PDF] Trigonometric Identities (Revision : 1.4





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Trigonometric Identities(Revision: 1.4)1 Trigonometric Identities you must remember

The "big three" trigonometric identities are

sin

2t+ cos2t= 1(1)

sin(A+B) = sinAcosB+ cosAsinB(2) cos(A+B) = cosAcosB-sinAsinB(3) Using these we can derive many other identities. Even if we commit the other useful identities to memory, these three will help be sure that our signs are correct, etc.

2 Two more easy identities

From equation (1) we can generate two more identities. First, divide each term in (1) by cos

2t(assuming it is not zero) to obtain

tan

2t+ 1 = sec2t.(4)

When we divide bysin2t(again assuming it is not zero) we get

1 + cot

2t= csc2t.(5)

3 Identities involving the difference of two angles

From equations (2) and (3) we can get several useful identities. First, recall that cos(-t) = cost,sin(-t) =-sint.(6)

From (2) we see that

sin(A-B) = sin(A+ (-B)) = sinAcos(-B) + cosAsin(-B) which, using the relationships in (6), reduces to sin(A-B) = sinAcosB-cosAsinB.(7)

In a similar way, we can use equation (3) to find

cos(A-B) = cos(A+ (-B)) = cosAcos(-B)-sinAsin(-B) which simplifies to cos(A-B) = cosAcosB+ sinAsinB.(8) Notice that by remembering the identities (2) and (3) you can easily work out the signs in these last two identities. 1

4 Identities involving products of sines and cosines

If we now add equation (2) to equation (7)

sin(A-B) = sinAcosB-cosAsinB +(sin(A+B) = sinAcosB+ cosAsinB) we find sin(A-B) + sin(A+B) = 2sinAcosB and dividing both sides by 2 we obtain the identity sinAcosB=12sin(A-B) +12sin(A+B).(9)

In the same way we can add equations (3) and (8)

cos(A-B) = cosAcosB+ sinAsinB +(cos(A+B) = cosAcosB-sinAsinB) to get cos(A-B) + cos(A+B) = 2cosAcosB which can be rearranged to yield the identity cosAcosB=12cos(A-B) +12cos(A+B).(10) Suppose we wanted an identity involvingsinAsinB. We can find one by slightly modi- fying the last thing we did. Rather than adding equations (3) and (8), all we need to do is subtract equation (3) from equation (8): cos(A-B) = cosAcosB+ sinAsinB -(cos(A+B) = cosAcosB-sinAsinB)

This gives

cos(A-B)-cos(A+B) = 2sinAsinB or, in the form we prefer, sinAsinB=12cos(A-B)-12cos(A+B).(11)

5 Double angle identities

Now a couple of easy ones. If we letA=Bin equations (2) and (3) we get the two identities sin2A= 2sinAcosA,(12) cos2A= cos2A-sin2A.(13) 2

6 Identities for sine squared and cosine squared

If we haveA=Bin equation (10) then we find

cosAcosB=12cos(A-A) +12cos(A+A) cos

2A=12cos0 +12cos2A.

Simplifying this and doing the same with equation (11) we find the two identities cos

2A=12(1 + cos2A),(14)

sin

2A=12(1-cos2A).(15)

7 Identities involving tangent

Finally, from equations (2) and (3) we can obtain an identity fortan(A+B): tan(A+B) =sin(A+B)cos(A+B)=sinAcosB+ cosAsinBcosAcosB-sinAsinB. Now divide numerator and denominator bycosAcosBto obtain the identity we wanted: tan(A+B) =tanA+ tanB1-tanAtanB.(16) We can get the identity fortan(A-B)by replacingBin (16) by-Band noting that tangent is an odd function: tan(A-B) =tanA-tanB1 + tanAtanB.(17)

8 Summary

There are many other identities that can be generated this way. In fact, the derivations above are not unique - many trigonometric identities can be obtained many different ways. The idea here is to be very familiar with a small number of identities so that you are comfortable manipulating and combining them to obtain whatever identity you need to. 3quotesdbs_dbs14.pdfusesText_20
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