- #1

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## Homework Statement

[tex]f(x, y) = \tan(x + y) \\

f_x = ?

[/tex]

## Homework Equations

[tex]\frac{dy}{dx}\tan(x)= \sec^2 x[/tex]

## The Attempt at a Solution

I set y as constant, so I said derivative of y = 0 then took derivative of tan as above. However the answer should be

[tex]f_x = \sec^2(x + y)[/tex]

Why is y included?