Question 5 (v) Prove the following identities, where the angles involved are acute angles for which the expressions are defined. (v) (cosA−sinA+1)(cosA+sinA−1)=cosecA+cotA, using the identity cosec2A=1+cot2A.
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Solution
(v)(cosA−sinA+1)/(cosA+sinA−1)=cosecA+cotAL.H.S.=(cosA−sinA+1)(cosA+sinA−1)Dividing Numerator and Denominator by sin A,=(cosA−sinA+1)sinA(cosA+sinA−1)sinA=(cotA−1+cosecA)(cotA+1−cosecA)=(cotA−cosec2A+cot2A+cosecA)(cotA+1−cosecA)(∵cosec2A−cot2A=1)=[(cotA+cosecA)−(cosec2A−cot2A)](cotA+1−cosecA)=[(cotA+cosecA)−(cosecA+cotA)(cosecA−cotA)](1−cosecA+cotA)=(cotA+cosecA)(1−cosecA+cotA)(1−cosecA+cotA)=cotA+cosecA=R.H.S.