① Make a straight line Mn on the other side different from point A;

② Observe AB ⊥ manganese;

③ Take two points C and D on the straight line MN to make BC = CD.

(4) The crossing of D is DE⊥MN, and the three-point * * * line of A, C and E is made, and the length of DE is river width.

(2) In Rt△ABC and Rt△EDC,

∠ABC=∠EDC=90，BC=CD，∠ACB=∠ECD，

∴Rt△ABC≌Rt△EDC.

∴AB=ED.

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