a man launches his boat from pt. a on a bank of a straight river, 3km wide, and wants to reach pt. b, 8km downstream on the opposite bank as quickly as possible. if he can row 6 km/hr and run 8 km/hr, where should he land to reach b as soon as possible?



Answer :

Let x = the distance between C and D, or 0x1.

We wish to cut down on the amount of time, t, that is required to row to point D and then run the remaining distance to point B.

As a right triangle, ACD. Let s equal the distance between A and D.

s=√(x2+42)

The distance from point D to point B is 1-x.

time equals distance/rate

t = √(x2+42)/6 + (1-x)/8

Take dt/dx, set it to zero, then solve for x to reduce t.

dt/dx is equal to x/(6x2+42) - 1/8.

x/(6√(x2+42)) = 1/8

(6/8)√(x2+42)=x

both sides are square.

(9/16)(x2+42)=x2

(9/16)x2+9 = x2

(7/16)x2=9

x = ±√((9)(16)/7) = ±12/√7 Neither value falls within the range [0, 1].

Verify the endpoints x=0 and x=1.

t(1) = √17/6 - 1/8 = 0.562

t(0) = 4/6+1/8 = 0.792

Therefore, x=1 reduces the time. The ideal course of action is to row directly to point B.

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