modulus function

diff --git a/spec/prelim.md b/spec/prelim.md
index 586fc59e6d0ca5ad207c321cb3c20e159d9a2263..ed06b09db8ad416577a72c0d21ee35889b9363de 100644 (file)
--- a/spec/prelim.md
+++ b/spec/prelim.md
@@ -63,3 +63,6 @@ $${(x-h)^2 \over a^2} - {(y-k)^2 \over b^2} = 1$$
 
 ${(x-h)^2 \over a^2} - {(y-k)^2 \over b^2} = 1$ and ${(y-k)^2 \over b^2} - {(x-h)^2 \over a^2} = 1$ are **conjugate hyperbolas**
 
+## Modulus function
+
+$$|x|=\sqrt{x^2}$$
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