1 times 21 is 21, but what is the answer if both 1 and 21 are negative numbers? Here we will determine if the answer to negative 1 times negative 21 is positive 21 or negative 21.
There are many different ways to explain and determine if the answer to negative 1 times negative 21 is -21 or +21 using mathematical terms such as additive inverse and mathematical laws like the Distribute Law.
We will take a different approach - process of elimination: We will simply test if -21 is the correct answer using basic algebra.
We will test if -21 is correct by setting up the equation negative 1 times negative 21 equals negative 21, and then simplifying the equation to see if it is true.
(-1) (-21) = (-21)
Divide both sides of the equation by (-21) :
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| = |
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The numerator and denominator on the left side both have (-21) which even each other out and can be removed:
| (-1) = |
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The numerator and denominator on the right side both have a minus sign (-) which even each other out and can be removed:
| (-1) = |
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A numerator of 21 and denominator of 21 on the right side is the same as 21 divided by 21 which is 1. Now we end up with this untrue equation:
(-1) = (1)
-1 is NOT equal to 1, thus we have proven that negative 1 times negative 21 is not equal to -21. The answer must therefore be 21:
-1 x -21 = 21
The above applies to all math problems where you multiply two negative numbers. Multiplying two negative numbers such as negative 1 times negative 21 will always have a positive answer.
Negative Times Negative Calculator
Here you can multiply another two negative numbers together.
What is negative 1 times negative 22?
Here is the next problem on our list that we have explained and calculated.
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