關於一題工程數學的問題

1. Assume that the uniform ice ball has a volume 1000cm^3, its melting rate is proportional to its surface area, After one minute , the volume of the ice ball decreased to be 729cm^3, How long will it takes that the volume is 125cm^3?


2. The ODE equation: [ sin (x + 2y) + 3x cosx (x + 2y) ] dx + 6x cosx (x + 2y) dy = 0
(a) Verify the ODE is not exact.
(b) Find the integrating factor I ( x , y ).
(c) Find the solution of the ODE.






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