1)
Why does the Torah need to add that Machaneh Yisrael stretched "from Beis Hayeshimos until Aveil ha'Shitim"?
1.
Rashi: To teach us that the Camp of Yisrael was twelve Mil (three Parsah) long.
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1
Rashi: As Rabah bar bar Chanah, who saw the area, testified - in Eruvin, 55b. Refer also to 9:23:1:3 and note.
2)
What is the meaning of "Aveil ha'Shitim"?
1.
Rashi: It means 'Aveil' (the name of the plain) of Shitim.
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2.
Targum Onkelos and Targum Yonasan: It means 'the plains of Shitim'.
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2
Since both 'Aveil' and 'Arvos' (the next word) mean 'plains'.