Prove that cos(x + y) =cos x cos y -sinx sin y .
Answers
Answered by
1
Recall the identity
cos
(
A
+
B
)
=
cos
A
cos
B
−
sin
A
sin
B
Putting
A
=
y
and
B
=
x
−
y
, we get
A
+
B
=
y
+
x
−
y
=
x
and the above identity becomes
cos
y
cos
(
x
−
y
)
−
sin
y
sin
(
x
−
y
)
=
cos
x
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