The Addition Sign or "Plus Sign" (+) is often more implied than shown. For example, stores list individual prices, with the implication that customers know that they must add prices together.
The "plus" sign is also shown on:
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There are many parts that go into an addition problem. Most often there are numbers, a plus sign, and an equals sign.
The less than sign I believe that you are talking about is the minus/subtraction sign? The addition sign, +, is the opposite of the subtraction sign :)
Because a negative negative is the same a a positive, and because of the definition of subtraction and addition.
You can indicate addition by using it's sign "+" or saying add/plus.
The guy who invented the addition sign