`tan(x+y) = x`

`or, sec^2(x+y)*[1 + (dy/dx)] = 1`

`or, dy/dx = [1-sec^2(x+y)]/{sec^2(x+y)}`

` `

``Now, dy/dx at (0,0) we get

`dy/dx = {1-sec^2(0+0)}/{sec^2(0+0)}`

`or, dy/dx = (1-1)/1 = 0`

``

`tan(x+y) = x`

`or, sec^2(x+y)*[1 + (dy/dx)] = 1`

`or, dy/dx = [1-sec^2(x+y)]/{sec^2(x+y)}`

` `

``Now, dy/dx at (0,0) we get

`dy/dx = {1-sec^2(0+0)}/{sec^2(0+0)}`

`or, dy/dx = (1-1)/1 = 0`

``