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## Homework Statement

You have two parameters x = 12 and y = 3 set on a machine. The machine generates a function: z (x, y) = 3sin (x ^ 2 + y) y + x ^ 3

Use the total differential of this function in the set point to determine which of the parameters to be set to the most accurate.

## Homework Equations

dz = (∂z/∂x) dx + (∂z/∂y) dy

3.Solution

z = 3y sin(x²+y) + x^3

[/B]

dz = (3y cos(x²+y) * 2x + 3x²) dx + (3 sin(x²+y) + 3y cos(x²+y) * 1 + 0) dy

dz = (6xy cos(x²+y) + 3x²) dx + (3 sin(x²+y) + 3y cos(x²+y)) dy

I don't understand why

**(∂z/∂y) = (3 sin(x²+y) + 3y cos(x²+y) * 1 + 0) dy Where did that zero came from ??? and 1 ??**