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Let us t  , from an initial condition (2.39) we find
С
10  , that is C  1000 . Therefore the law of change of
100
amount of salt x in kilograms, that is in a reservoir, depending
on the passed time in minutes (rice. 2.1б) is set by a formula

1000
x  . (2.40)
(10 t ) 2

We will notice that from a formula (2.40), knowing the
amount of salt, that remained in a reservoir (last it is easily to
set, measuring the volume of reservoir and concentration of salt
in him), it is possible to define the amount of time, which passed
from the beginning of process. On this idea the calculation of
age of seas and oceans is founded. 

Exercises:

1. For each of the following equations to set argued his
type and method of decision :

) a e y (1 x 2 )dy  2 (1x  e y )dx  0 ;
y y
б ) (x  y cos )dx  x cos dy  0 ;
x x
3
3
2
в ) (3xy  2 )x dx  y dy  0 ;
г ) cos x y ' sin x y   1 ;
2
д ) ( y  ) x dx  2xydy  0 ;
y 3 4
) е ' y   x y ;
x
є ) (1 y 2 )xdx  (1 x 2 )dy  0 ;
ж ) (x  y ) ' (y  x  ) y  0 .

2. To find the common decision of equation :
2
2
а ) (x  2xy )dx  (x  y 2 )dy  0
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