Example 2:
Prove each of the following trigonometric identities.
(a)1+cos2xsin2x=cotx(b)cotAsec2A=cotA+tan2A(c)sinx1−cosx=cotx2
Solution:
(a)
LHS=1+cos2xsin2x=1+(2cos2x−1)2sinxcosx=2cos2x2sinxcosx=cosxsinx=cotx=RHS(proven)
(b)
RHS=cotA+tan2A=cosAsinA+sin2Acos2A=cosAcos2A+sinAsin2AsinAcos2A=cosA(cos2A−sin2A)+sinA(2sinAcosA)sinAcos2A=cos3A−cosAsin2A+2sin2AcosAsinAcos2A=cos3A+cosAsin2AsinAcos2A=cosA(cos2A+sin2A)sinAcos2A=cosAsinAcos2A←sin2A+cos2A=1=(cosAsinA)(1cos2A)=cotAsec2A
(c)
LHS=sinx1−cosx=2sinx2cosx21−(1−2sin2x2)←sinx=2sinx2cosx2,cosx=1−2sin2x2=2sinx2cosx22sin2x2=cosx2sinx2=cotx2=RHS(proven)