Fill in the missing number:
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Fill in the missing number:
Remember the following law:
Let's think about which number we need to multiply by 7 in order to get 21:
Now let's put the numbers together with the appropriate sign as written in the law above as follows:
Solve the following exercise:
\( (+6)\cdot(+9)= \)
Because we need a negative product (-21)! When one factor is positive and the other is negative, the result is always negative. Since 7 is positive, the missing factor must be negative.
Think of it like this: same signs give positive (+ × + or - × -), and different signs give negative (+ × - or - × +). Since we need -21, we need different signs!
Check your work! If you multiply 7 × 3, you get +21, not -21. The question asks for -21, so you need the negative factor: .
Yes! Try division: . Division and multiplication are inverse operations, so this confirms your answer.
Let's test it: , not -21! Always substitute your answer back into the original equation to verify it works.
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