Simplify the following expressions

a) cos(180° + x) + cos(180° - x)

b) cos(126°)cos(36°) + sin(126°)sin(36°)
can you please show me how to do this

1 answer

You must have been studying the identity:
cos(A+B) = cosAcosB - sinAsinB
cos(A-B) = cosAcosB + sinAsinB

so in a) apply both of those, and knowing that
cos 180° = -1 and sin 180° = 0
simplify

for b)
compare cos(126°)cos(36°) + sin(126°)sin(36°)
with cos(A-B) = cosAcosB + sinAsinB

let me know what you got