Make sure you can plot graphs of linear equations and recognise the equation of a line from a graph before looking at graphs of inequalities.

The graph below shows the total number of publications each year in Inequalities and Differential Equations on Lie Groups and Manifolds.

The graph below shows the total number of publications each year in Dynamic Inequalities and Integral Equations on Time Scales.

H. W. Kuhn, Solvability and Consistency for Linear Equations and Inequalities, The American Mathematical Monthly, Vol. 63, No. 4 (Apr., 1956), pp. 217-232 ...