literal equations / exponential modelling

diff --git a/methods/stuff.md b/methods/stuff.md
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+++ b/methods/stuff.md
@@ -57,3 +57,18 @@ Inverse of $f: \mathbb{R} \rightarrow \mathbb{R}, f(x)=a^x$ is $f^{-1}: \mathbb{
 ## Euler's number
 
 $$e= \lim_{n \rightarrow \infty} (1 + {1 \over n})^n$$
 ## Euler's number
 
 $$e= \lim_{n \rightarrow \infty} (1 + {1 \over n})^n$$

+
+## Literal equations
+
+_Literal equation_ - no numerical solutions
+
+## Exponential and logarithmic modelling
+
+$$A = A_0 e^{kt}$$
+
+where  
+$A_0$ is initial value  
+$t$ is time taken  
+$k$ is a constant  
+For continuous growth, $k > 0$  
+For continuous decay, $k < 0$