Question

ans this...................
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Answer 1

You can not cancel 6 in the first two cases.

You can cancel 2 in the third case as you have taken 2 common from the whole thing.

You have taken 2 common from the whole numerator. So 2 is multiplied with the entire thing in numerator.
Similarly, in denominator, 2 is multiplied with the entire thing.
Now you can cancel 2.

In the first two cases, 6.was not multiplied with the entire numerator. So you can't cancel 6 there.