First, here is a simple example:
Just multiply numerator by numerator and denominator by denominator. So you get:
And this fraction can be cancelled: .
BTW it is important to keep the following stuff in mind: If you multiply fractions, you multiply the numerators and the denominators. But if you add or subtract fractions, you leave the denominator the same (after making the fractions of the same denominator)! Keep that in mind!
Though the denominators are the same, you have to multiply numerators and denominators. This gives:
In this case, one has to work with quite large numbers. This can be avoided by cancelling over cross. You are allowed to cancel numbers and .
Now we see that and are both divisible by , so you can cancel by :
Though this exercise looked complicated at first, you only have to multiply small numbers.
Important: This cancelling over cross only works fur multiplications! It is always wrong to do this for addition / subtraction exercises.
Another example involving Division:
To divide by a fraction, multiply by the inverse fraction. This means:
Now you can cancel over cross:
equals (by the way)
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