1.

If \(\rm z = \cos \left( {\frac{x}{y}} \right) + \sin \left( {\frac{x}{y}} \right)\)then \(x\frac{{\partial z}}{{\partial x}} + y\frac{{\partial z}}{{\partial y}} = \)

A. 1
B. cos x
C. sin y
D. 0
Answer» E.


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