3. Integrable Combinations
There are many differential equations where we cannot separate the variables, like we saw in the previous section.
However, we can possibly solve the DE if we use one of the following expressions to get the differential equation in a form that we can solve:
(1) `d(xy) = x\ dy + y\ dx`
(2) `d(x^2+ y^2) = 2(x\ dx + y\ dy)`
(3) `d(y/x)=(x\ dy-y\ dx)/x^2`
(4) `d(x/y)=(y\ dx-x\ dy)/y^2`
These differential forms are called integrable combinations. If we can transform our differential equation into one side of one of the above forms, then we can use the other side to solve the DE.
Where do these integrable combinations come from?
One way to think of the first expression above is as follows. Say x and y are both functions of t. If we have the product xy, then the derivative with respect to t of that product would be given by:
(This is just the product rule that we learned ages ago.) Then we can multiply throughout by dt and we get the expression in (1) above, `d(xy) = x\ dy + y\ dx`.
The third and fourth combinations are based on the Quotient Rule.
(2y + x)dy + y dx = 0
x dx = 9x2 dx − y dy
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