If \[x=\cos \theta +i\sin \theta \] and \[y=\cos \varphi +i\sin \varphi \],then \[{{x}^{m}}{{y}^{n}}+{{x}^{-m}}{{y}^{-n}}\] is equal to

If \[x=\cos \theta +i\sin \theta \] and \[y=\cos \varphi +i\sin \varph

1.	If \[x=\cos \theta +i\sin \theta \] and \[y=\cos \varphi +i\sin \varphi \],then \[{{x}^{m}}{{y}^{n}}+{{x}^{-m}}{{y}^{-n}}\] is equal to
A.	\[\cos (m\theta +n\varphi )\]
B.	\[\cos (m\theta +n\varphi )\]
C.	\[2\cos (m\theta +n\varphi )\]
D.	\[2\cos (m\theta -n\varphi )\]
Answer» D. \[2\cos (m\theta -n\varphi )\]