to ensure and re-check your work you use the opposite signs (addition & subtractions, multiplication & division)
e.g.
__= blank
10+ _ = 12
_=2
to check: you take the answer (in this case 12) and subtract (because its the opposite of addition) by the blank you filled in (2 in this case.
therefore, 12-2=10 so we know its correct :-)
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Subtraction is used to check addition.
Addition is the opposite of subtraction. For example, 24 - 9 = 15. If you want to check your answer, add 9 + 15. It equals 24, so your answer is correct.
addition and subtraction
for addition you take the sum and subtract one of the numbers being added from it and for subtraction you add the difference with the second number being subtracted so it equals the first number
d-a-s division-addition-subtraction