Tinh dk $\lambda =30cm$
Hai nguon nguoc pha nen tren mat nc se co 7 duong dao dong cuc tieu la trung diem cua $AB$
va 6 duong con lai chia deu ve hai phia cua duong trung truc.
De $BM$ nho nhat thi $M$ thuoc duong cong thu 3 tuc la:
$AM-BM=3\lambda $
$<=>\sqrt{BM^2+AB^2}-BM=90$
$<=>\sqrt{BM^2+100^2}-BM=90$
$<=>BM\approx 10,56cm$