Question

sin780×sin480+cos240×cos300=1/2​

Answer 1

Step-by-step explanation:

)sin780.sin120+cos240.sin390 

=sin(8x90+60).sin(90+30) +cos(3x90-30).sin(4x90+30) 

=sin60.cos30+[-sin30].sin30 

=sin60.cos30 - sin30.sin30 

=(root3)/2 x (root3)/2 -1/2 x1/2 

=3/4 -1/4 

=2/4 

=1/2(proved)