(2): From Pythagorean Theorem, AB? =AC？ +BC？ So, AB= AC under the root sign? +BC？ Under the root sign = 2. 1? +2.8? =3.5
(3): the intersection point C is CD⊥AB and the vertical foot D.
With the same area of AB×CD=AC×BC, 3.5×CD=2. 1×2.8, CD= 1.68.
10 solution: let the depth of water be x feet and the length of reed be (x+ 1). According to Pythagorean Theorem, X? ( 10/2)? =(x+ 1)？
Solution: x= 12, so x+ 1= 13 (feet).
Answer: water depth 12 feet, reed length 13 feet.