Question

write the given expression as the cosine of an angle:
cos(70°)cos(65°)-sin(70°)sin(65°)

a)cos(65°)
b)cos(135°)
c)cos(5°)
d)cos(70°)
e)cos(-130°)

Answer 1

Know that cos(x)=sin(90-x)
likewise sin(x)=cos(90-x)

Answer 2

ya I got that.... just don't know where to go from there

Answer 3

so just change sin() into cos()

Answer 4

ok but thats still not one of the choices for an answer

Answer 5

cos(70)cos(65)-cos(20)cos(35)

Answer 6

When you solve this you should get numerical answer of about -.625
Take arcos of this then you will get 128. which is close to 135

Answer 7

ok... thanks