Hehe, you know that the form of writing a paper is not simple, and the format is different. This topic is not difficult. In fact, middle school knowledge is enough. It is the shortest straight line between two points. Since it is a lake, let's assume that the water is still.

The picture is not easy to insert, so you can only read it yourself.

Set the total time to t, and let her walk in a straight line whether running or swimming.

T= (root number (x 2+2 2))/18+(root number (3 2+(7-x) 2))/6

There is only one problem left, to find the maximum value.

Take the derivative so that its derivative function is 0.

Solve the equation 8x4-112x3+347x2-504x+1764 = 0.

Solve with matlab and input the program.

p=[8，- 1 12，347，-504， 1764]；

Px=poly2str(p，' x ')；

Format ratio

& gt& gtr = root (p)

Get the result:

r =

4339/429

1720/4 13

-390/2797 + 8 13/356i

-390/2797 - 8 13/356i

Discard virtual roots and roots greater than 7.

Get: 1720/4 13

Keep decimals again

Enter x =1720/413;

vpa(x，3)

Enter to get ans =

4. 16

That is to say, it is 2 kilometers due south of A and 4. 16 kilometers due east.