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(*CreatetheCSVfilewithafirstline*)

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data1={{1,2,3,4,5}};

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Export["output.csv",data1];

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(*HowtoaddasecondlinetoCSV,butwithouttheBRACES{}?*)

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data2={{6,7,8,9,10}};

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str=OpenAppend["output.csv"];Write[str,data2];Close[str];