It belongs to equivalent substitution, because the idea of equivalent substitution is embodied by the nature of the equation, which is transitive: if a = b and b = c, then a = C. Angle 1 equals 10, and angle 2 equals 10, so angle 1 equals angle 2.

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Definition of equivalent substitution: substituting one quantity (or part of another quantity) for another quantity (or part of another quantity) equal to it. "Equivalent substitution" means that a quantity is replaced by an equivalent quantity. It is a basic thinking method in mathematics and the basis of algebraic thinking method. The real equivalent substitution is:? F(a=b∧f(a)→f(b)), where f is the generalized equivalent substitution of the formula.