Question

What is the exact value of cos 225° cos 75°?

Answer 1

by using identity
cos a cos b = (1/2) [cos (a - b) + cos (a + b)]
cos225cos75=1/2[cos(225-75)+cos(225+75)]
=1/2[cos150+cos300]
=1/2[-0.86+0.5]
=1/2[-0.36]
=-0.18

Answer 2

so (1-sqrt3)/4?

Answer 3

thank you