c
o
s
(
α
+
β
)
=
c
o
s
(
α
)
c
o
s
(
β
)
−
s
i
n
(
α
)
s
i
n
(
β
)
c
o
s
(
α
−
β
)
=
c
o
s
(
α
)
c
o
s
(
β
)
+
s
i
n
(
α
)
s
i
n
(
β
)
c
o
s
(
α
+
β
)
=
c
o
s
(
α
)
c
o
s
(
β
)
−
s
i
n
(
α
)
s
i
n
(
β
)
c
o
s
(
α
−
β
)
=
c
o
s
(
α
)
c
o
s
(
β
)
+
s
i
n
(
α
)
s
i
n
(
β
)
Close
Log in
Books
Repository
eAnswer
ePortfolio
Close
Log in
Books
Repository
eAnswer
ePortfolio
Log in
New user
OK
Error
Forgot?
Log in
Register
Recover
OK
Error
Change