Division: The greatest common divisor of two integers is equal to the greatest common divisor of the difference between the smaller number and the two numbers. In this way, these two numbers keep decreasing until one of them becomes zero.

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The greatest common divisor of 1)49 and 2 1 is that of 2 1 and 49-2 1=28.

2) The greatest common divisor of 2 1 and 28 is the common divisor of 21and 28-2 1=7.

3) The greatest common divisor of 21and 7 is the common divisor of 7 and 2 1-7= 14.

4) The greatest common divisor of 7 and 14 is that of 7 and 14-7=7.

5) The greatest common divisor of 7 and 7 is 7.

Or 5) The greatest common divisor of 7 and 7 is the greatest common divisor of 7 and 7 -7=0, which is 7.