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One of the basic arithmetic operations. The result of addition is called the sum. The sum of two numbers $a$ and $b$ is denoted by $a+b$, and $a$ and $b$ are called the summands. Addition of numbers is commutative: $a+b=b+a$, and associative: $(a+b)+c=a+(b+c)$. The operation inverse to addition is called subtraction.