[spec] DEs of the dependent variable

diff --git a/spec/calculus.md b/spec/calculus.md
index c6818d4a70fc710c8ac1f5a7ef5e08173d0b718e..30a295788bca6e503648f0a6a247a7cbd3957882 100644 (file)
--- a/spec/calculus.md
+++ b/spec/calculus.md
@@ -396,3 +396,7 @@ e.g. ${\left(dy^2 \over d^2 x\right)}^3$: order 2, degree 3
 ### Verifying solutions
 
 Start with $y=\dots$, and differentiate. Substitute into original equation.
+
+### Function of the dependent variable
+
+If ${dy \over dx}=g(y)$, then ${dx \over dy} = 1 \div {dy \over dx} = {1 \over g(y)}$. Integrate both sides to solve equation. Only add $c$ on one side. Express $e^c$ as $A$.