# Year 2015 in Review – The Most Popular Math Posts

It’s the end of the year again! A lot of things has happened this year including the **death** of brilliant mathematician John Nash.

It has always been a tradition to look at the popular posts at the end of the year, so let’s look at what we have learned in 2015.

**Year 2015 – Most Popular Math Posts**

- How to Use the Factorial Notation
- What exactly is the vertical line test?
- What is the horizontal line test?
- Understanding the Concept of Inverse Functions
- Why the Area of a Rhombus is Half the Product of its Diagonals
- Why the Multiplication by 11 Trick Works
- 4 Interesting Things about the Japanese Counting Systems
- How to Represent Inequalities in One Variable
- A Different Way to Perform Division
- The Lattice Multiplication Algorithm
- How to Add Integers Using the Number Line
- Proof Without Words: If x >0, x + 1/x >=2
- The Basics of Inverse Proportion
- The Basics of Direct Proportion
- Increasing and Decreasing a in the linear function y = ax
- The effect of the sign of the slope in y = ax
- The effect of adding b to the linear function y = ax + b
- Properties and Graph of Linear Functions
- Derivation of the Area of a Rhombus
- A Closer Look at Coinciding Lines

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