The horizontal line above the word C-not C, different from C; A horizontal line above R-non-R, if R represents a group of real numbers, non-R refers to non-real numbers; The word A-Δ A on the side of the triangle is capitalized in Greek letters and pronounced as "Delta". Δ A represents the variation of A, or the increment of A, for example, the original value A0 of A =1=18, then

δa = a 1-A0 = 1. 18- 1 = 0. 18 .