The equation of tangents to the curve `y=cos(x+y), -2pi<=x<=2pi` that are parallel to the line x +2y= 0

Explanation

`x+2y=0`

`m=-1/2`

`y=cos(x+y)`

differentiate with respect to x

`dy/dx=-sin(x+y)(1+y^2)`

`-1/2=-sin(x+y)(1-1/2)`

`-1=-sin(x+y)`

`sin(x+y)=1`

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