The figure on the right (figure 3), shows the consumption patterns of the consumer of two goods X1 and X2, the prices of which are p1 and p2 respectively, where B1 and B2 are the budget lines and I1 and I2 are the indifference curves.
\Delta X_n^1 is the change in the demand for good 1 when we change income from m' to m, holding the price of good 1 fixed at p_1: .