In short
A linear equation is an equation in which each variable has power 1, such as . Its graph is a straight line.
An equation in one variable, such as , has one solution. An equation in two variables has infinitely many solutions, which are the points of a line. Two linear equations in two variables, solved together, are simultaneous equations. They can be solved by substitution, by elimination or by graphs. Their lines may cross at one point, be parallel (no solution), or lie on top of each other (infinitely many).
and . Adding gives , so and . The lines cross at .
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