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[Solved] Trigonometric equations

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The second equation implies  \small 4y^3+sinycosy=-a.

Multiply the equation by \small -2 to obtain \small (-2y)^3+sin(-2y)=2a.

The first equation implies \small x^3+sinx=2a,  then  \small f(x)=f(-2y).

Let \small f(t)=t^3+sint. Since the function \small f(t) is increasing where \small t\epsilon [-\frac{\Pi }{2},\frac{\Pi }{2}]\Rightarrow x=-2y, 

therefore \small x+2y=0.

As a conclusion, \small cos(x+2y)=1.

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