Answer:
no
Step-by-step explanation:
yes
Answer:
and 
Step-by-step explanation:
Recall that
is a value of the function cosine for the special angles:
and
, then:

Answer:
b
Step-by-step explanation:
Hi there!
To solve is pretty easy. First, simplify.
21/4=(1/2x+3x) -> Combine Like Terms
21/4=7/2x
FLIP
7/2x=21/4
Multiply both sides by 2/7.
(2/7)*(7/2x)=(2/7)*(21/4)
x = 2/3
Hope this helps!
You would set the equations equal to eachother and get x=-5/4 and then plug that in for x, i hope this helps!