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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