where μ is a constant, is reducible to a standard form by taking a new independent variable z = ∫ dx[φ(x)]-½.
We pass to other types of equations of which the solution can be obtained by rule. We may have cases in which there are two dependent variables, x and y, and one independent variable t, the differential coefficients dx/dt, dy/dt being given as functions of x, y and t. Of such equations a simple case is expressed by the pair
| dx | = ax + by + c, | dy | a′x + b′y + c′, |
| dt | dt |
wherein the coefficients a, b, c, a′, b′, c′, are constants. To integrate these, form with the constant λ the differential coefficient of z = x + λy, that is dz/dt = (a + λa′)x + (b + λb′)y + c + λc′, the quantity λ being so chosen that b + λb′ = λ(a + λa′), so that we have dz/dt = (a + λa′)z + c + λc′; this last equation is at once integrable in the form z(a + λa′) + c + λc′ = Ae(a + λa′)t, where A is an arbitrary constant. In general, the condition b + λb′ = λ(a + λa′) is satisfied by two different values of λ, say λ1, λ2; the solutions corresponding to these give the values of x +λ1y and x + λ2y, from which x and y can be found as functions of t, involving two arbitrary constants. If, however, the two roots of the quadratic equation for λ are equal, that is, if (a − b′)² + 4a′b = 0, the method described gives only one equation, expressing x + λy in terms of t; by means of this equation y can be eliminated from dx/dt = ax + by + c, leading to an equation of the form dx/dt = Px + Q + Re(a + λa′)t, where P, Q, R are constants. The integration of this gives x, and thence y can be found.
A similar process is applicable when we have three or more dependent variables whose differential coefficients in regard to the single independent variables are given as linear functions of the dependent variables with constant coefficients.
Another method of solution of the equations
dx/dt = ax + by + c, dy/dt = a′x + b′y + c′,
consists in differentiating the first equation, thereby obtaining
| d²x | = a | dx | + b | dy | ; |
| dt² | dt | dx |
from the two given equations, by elimination of y, we can express dy/dt as a linear function of x and dx/dt; we can thus form an equation of the shape d²x/dt² = P + Qx + Rdx/dt, where P, Q, R are constants; this can be integrated by methods previously explained, and the integral, involving two arbitrary constants, gives, by the equation dx/dt = ax + by + c, the corresponding value of y. Conversely it should be noticed that any single linear differential equation