1/a \!\(\*SubsuperscriptBox[\(\[Integral]\), \(0\), \(a\)]\(x \*SuperscriptBox[\(Sin[\*FractionBox[\(\[Pi]\), \(a\)] x]\), \(2\)]\)\) + x Sin[2 \[Pi]/a x]^2 + 2 Sin[\[Pi]/a x] Sin[2 \[Pi]/a x] Cos[3 \[Omega] t] \[DifferentialD]xIntegrate::nodiffd: \!\(\*SubsuperscriptBox[\(\[Integral]\), \(0\), \(a\)]\(x Sin[\*FractionBox[\(\[Pi]\), \(a\)] x]\)\) cannot be interpreted. Integrals are entered in the form \[Integral]f \[DifferentialD]x, \!\(\*SubsuperscriptBox[\(\[Integral]\), \(a\), \(b\)]\(f \[DifferentialD]x\)\), or Underscript[\[Integral], vars\[Element]region]f, where \[DifferentialD] is entered as \[EscapeKey]dd\[EscapeKey].