sin2 α =2*sin α cos α
поэтому
sin81^(o)*cos81^(o)=(1/2)sin162^(о)
sin162^(o)=sin(180^(o)-18^(o))= [b]sin18^(o)
[/b]
Формула понижения степени
cos^2 α =(1+cos2 α) /2
поэтому
cos^251^(o)=(1+cos102^(o))/2
cos(102^o)=cos(90^(o)+12^(o))= [b]-sin12^(o)[/b]
sin81^(o)*cos51^(o)*cos81^(o)*cos51^(o)=
=sin81^(o)*cos81^(o)*cos51^(o)*cos51^(o)=
=(1/2)sin162^(o)*(1+cos102^(o))/2=
=(1/4)*(sin18^(o)*(1-sin12^(o))=
=(1/4)*(sin18^(o)-sin18^(o)*sin12^(o))=
формула sin α *sin β =(1/2)cos( α - β)-(1/2)cos( α + β )
(1/4)*(sin18^(o) - (1/2)cos6^(o)+(1/2)cos30^(o))=
=(1/4)sin18^(o) - (1/8)cos6^(o)+(1/8)*sqrt(3)/2