Question

sin 130 cos 55 + cos 40 cos 125​

Answer 1

sin 130 cos 55 + cos 40 cos 125
=sin 130 cos 55 + cos 40 cos (180-55)
=sin 130 cos 55 + cos (90-50) (-cos 55) [cos (180-x)= -cos x]
=sin130 cos 55 - sin 50 cos 55 [cos (90-x)= sin x]
=cos 55 (sin 130 - sin 50)
=cos 55 ( sin (180-50) -sin 50) [sin (180-x)= sin x]
=cos 55 (sin 50 - sin 50)
=0