Answer :

Formula : -

(i) Cos 3 a = 4 cos^{3} a – 3 cos a

(ii) sin3a = 3sina – 4sin^{3}a

(iii) a^{3} – b^{3} = (a^{2} – b^{2})(a^{2} + b^{2} + ab)

(iv) sin^{2}a + cos^{2}a = 1

Taking L.H.S we get,

using formula (i) and (ii)

taking common (cos a – sin a) and using formula (iii)

using formula(iv)

L.H.S = R.H.S

Hence, Proved.

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