񂫪3(6/1/2001)

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   1. 
      

      $$yuu_x + xuu_y = xy$$ (13)

      
      

   2. 
      

      $$u_x + 3 u_y = 5 e^{2x+y}$$ (14)

      
      

   3. 
      

      $$u_x + 2 u_y + u = xy$$ (15)