Question

sin(-1071)×sin99+sin(-171)×sin（-261）等于?

Answer 1

sin(-1071)= -sin(-1071+5*180)= -sin(-171)

sin99=sin(99-2*180)=sin(-261)

因此

sin(-1071)*sin99+sin(-171)*sin(-261)

= -sin(-171)*sin(-261)+sin(-171)*sin(-261)

=0