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       椭园 oval(2,1)  | 
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   蝴蝶 with click parametric button and polarplot( )  | 
   
   
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`int _0^oo exp(-2x)` dx =  laplace(exp(-2x)) 
 然后设自变量x=0得到广义积分的值.
 
 `int _0^oo exp(-2x)` dx =  laplace(exp(-2x),x=0) 
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