Updated examples

docs/reference/arithmetic/abs.md

@@ -25,6 +25,8 @@ Number | BigNumber | Complex | Array | Matrix | Absolute val
 ## Examples
 
 ```js
+var math = mathjs();
+
 math.abs(3.5);                // returns Number 3.5
 math.abs(-4.2);               // returns Number 4.2
 

docs/reference/arithmetic/add.md

@@ -26,6 +26,8 @@ Number | BigNumber | Complex | Unit | String | Array &#
 ## Examples
 
 ```js
+var math = mathjs();
+
 math.add(2, 3);               // returns Number 5
 
 var a = math.complex(2, 3);

docs/reference/arithmetic/divide.md

@@ -26,6 +26,8 @@ Number | BigNumber | Complex | Unit | Array | Matrix |
 ## Examples
 
 ```js
+var math = mathjs();
+
 math.divide(2, 3);            // returns Number 0.6666666666666666
 
 var a = math.complex(5, 14);

docs/reference/arithmetic/multiply.md

@@ -26,6 +26,8 @@ Number | BigNumber | Complex | Unit | Array | Matrix |
 ## Examples
 
 ```js
+var math = mathjs();
+
 math.multiply(4, 5.2);        // returns Number 20.8
 
 var a = math.complex(2, 3);

docs/reference/arithmetic/subtract.md

@@ -26,6 +26,8 @@ Number | BigNumber | Complex | Unit | Array | Matrix |
 ## Examples
 
 ```js
+var math = mathjs();
+
 math.subtract(5.3, 2);        // returns Number 3.3
 
 var a = math.complex(2, 3);