Prove that 

(i) sin(A + B) sin(A – B) = sin² A – sin² B 

(ii) cos(A + B) cos(A – B) = cos² A – sin² B = cos² B – sin² A

(iii) sin²(A + B) – sin²(A – B) = sin2A sin2B 

(iv) cos 8θ cos 2θ = cos² 5θ – sin² 3θ

The structure of sodium chloride crystal is ………..

(A) body centered cubic lattice

(B) face centered cubic lattice

(C) octahedral

(D) square planar

नाखूनों की सफाई क्यों आवश्यक है?