`sin(n+1)xsin(n+2)x+cos(n+1)xcos(n+2)x=cosx`

1.	`sin(n+1)xsin(n+2)x+cos(n+1)xcos(n+2)x=cosx`
Answer» LHS `=cos(n+1)xcos(n+2)x+sin(n+1)xsin(n+2)x` `=cos{(n+1)x-(n+2)x}` `=cos(-x)=cosx` = R.H.S. Hence Proved.

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