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In South Korea, after six years of drilling addition, subtraction, multiplication, and division in elementary school, you enter middle school (equivalent to 7th~9th grades). There, you learn somewhat abstract mathematics for the first time and how to express unknown quantities in terms of alphabets such as x, y, and z. The first article talks about this abstraction. The second to fourth articles are about equations. Equations involve unknowns, and by solving them, you find the unknowns. Even though you may know how to solve systems of linear equations, I encourage you to read “Systems of Linear equations, part II” as I talk about an important concept called “linear independence” in the final comment there. The last article is about the Cartesian coordinate system, first introduced by and named after the great French philosopher René Descartes. (Yes, the one who famously said “I think, therefore I am.”) All the equations considered in these articles are linear equations. You will find the meaning of “linear” in our article “Polynomials, expansion and factoring’ in the “Nonlinear world’’ section.