Solve[lbk]{40*492
\!\(\*SuperscriptBox[lbk]\(x\), \(\[lbk]And[rbk]\)[rbk]\) 4 - (492*238145 +
492*16021 + 40*16021 + 142*2800*Cos[lbk]xy[rbk])
\!\(\*SuperscriptBox[lbk]\(x\), \(\[lbk]And[rbk]\)[rbk]\) 2 + (-40*2800
\!\(\*SuperscriptBox[lbk]\(x\), \(\[lbk]And[rbk]\)[rbk]\) 3 + 238145*2800 x)*
Sin[lbk]xy[rbk] + 238145*16021 == 0 && -40*142
\!\(\*SuperscriptBox[lbk]\(x\), \(\[lbk]And[rbk]\)[rbk]\) 3 - 492*142
\!\(\*SuperscriptBox[lbk]\(x\), \(\[lbk]And[rbk]\)[rbk]\) 3 + 142*2800
\!\(\*SuperscriptBox[lbk]\(x\), \(\[lbk]And[rbk]\)[rbk]\) 2*Sin[lbk]xy[rbk] +
238145*142 x + (-40*2800
\!\(\*SuperscriptBox[lbk]\(x\), \(\[lbk]And[rbk]\)[rbk]\) 3 + 238145*2800 x +
16021*2800 x)*Cos[lbk]xy[rbk] == 0}, {x, y}[rbk]

xy中间要有乘号*或者空格, 否则就是一个新的变量的名字