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partial derivative

The derivative of a function of several variables with respect to just one of them, every other variable held fixed at a constant value; written $\partial f/\partial w_1$ .

Examples

For $f(w_1,w_2)=w_1^2+2w_2^2$ , $\partial f/\partial w_1 = 2w_1$ , which is $2$ at $(1,1)$ .