If you read the title as trick equations, you are not wrong.
Procedure
Use identities, especially Pythagorean and double angle identities, to convert multiple trig functions into only sin or cos or tan. See list of identities at SL and identities at HL.
Get to a linear or quadratic equation in a single trig function
Get to a point where you can simply take the inverse trig function
When the argument is something like ω(x−h) with domain restriction a≤x≤b, then rewrite using ω(a−h)≤ω(x−h)≤ω(b−h), possibly flip the signs when ω<0.
Identify two solutions for ω(x−h) in different quadrants. Other solutions differ by 2nπ, ie co-terminal with the two.
Equation
Solutions for x
sinx=k
sin−1k+2nππ−sin−1k+2nπ
cosx=k
cos−1k+2nπ−cos−1k+2nπ
tanx=k
tan−1k+nπ
The solution for sin comes from the HL identity of sin(x)=sin(π−x).
The solution for cos comes from the HL identity of cos(x)=cos(−x).
The solution for tan comes from the fact that tangent is the slope of a line through the origin, and lines differing by an angle π radians have the same slope.
Example: Solve sin(2(x+3π))=−21,0≤x≤2π
Let y=2(x+3π).
2(0+3π)32π64π≤y≤y≤y≤2(2π+3π)≤314π≤328π
sin−1(−21)=−6π. The other base solution for y is π−(−6π)=67π. We need all the co-terminal angles in the new interval, by adding 2π=612π.
This means −6π,611π,623π and also 67π,619π. −6π can be rejected as it is outside the new interval.