Solve dy/dx=cos(x+y)

Solve the differential equation dy/dx=cos(x+y) by using the separation of variables (Fourier) method.
How to solve differential equation with separable variables?

Strategy of solving differential equations involving trigonometric functions by using separation of variables method, assuming θ = x+y:
1. let z = x+y and differentiate both sides with respect to x,
2. substitute accordingly,
3. apply trigonometric identities if needed and integrate both sides.

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#Differential #Equation