If f(x,y)=tan(x+y)f(x, y) = \tan(x + y)f(x,y)=tan(x+y), what is the first-order partial derivative fxf_xfx?
sec2(x+y)\sec^2(x + y)sec2(x+y)
cos2(x+y)\cos^2(x + y)cos2(x+y)
−csc2(x+y)-\csc^2(x + y)−csc2(x+y)
sec(x+y)tan(x+y)\sec(x + y)\tan(x + y)sec(x+y)tan(x+y)