Step 1 :
Divide $ 170 $ by $ 122 $ and get the remainder

$$ 170 : 122 = 1 \text{ ( remainder is } \color{blue}{ 48 } \text{ ) } $$

The remainder is positive ($ 48 > 0 $), so we will continue with division.

Step 2 :
Divide $ 122 $ by $ \color{blue}{ 48 } $ and get the remainder

$$ 122 : 48 = 2 \text{ ( remainder is } \color{blue}{ 26 } \text{ ) } $$

The remainder is still positive ($ 26 > 0 $), so we will continue with division.

Step 3 :
Divide $ 48 $ by $ \color{blue}{ 26 } $ and get the remainder

$$ 48 : 26 = 1 \text{ ( remainder is } \color{blue}{ 22 } \text{ ) } $$

The remainder is still positive ($ 22 > 0 $), so we will continue with division.

Step 4 :
Divide $ 26 $ by $ \color{blue}{ 22 } $ and get the remainder

$$ 26 : 22 = 1 \text{ ( remainder is } \color{blue}{ 4 } \text{ ) } $$

The remainder is still positive ($ 4 > 0 $), so we will continue with division.

Step 5 :
Divide $ 22 $ by $ \color{blue}{ 4 } $ and get the remainder

$$ 22 : 4 = 5 \text{ ( remainder is } \color{blue}{ 2 } \text{ ) } $$

The remainder is still positive ($ 2 > 0 $), so we will continue with division.

Step 6 :
Divide $ 4 $ by $ \color{blue}{ 2 } $ and get the remainder

$$ 4 : \color{blue}{ \boxed { 2 }} = 2 \text{ ( remainder is } \color{blue}{ 0 } \text{ ) } $$

The remainder is zero => GCD is the last divisor $ \color{blue}{ \boxed { 2 }} $.

This solution can be visualized using a Venn diagram.

The GCD equals the product of the numbers at the intersection.

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