Step 1 : Find prime factorization of each number.

$$\begin{aligned}987654 =& 2\cdot3\cdot97\cdot1697\\[8pt]123456 =& 2\cdot2\cdot2\cdot2\cdot2\cdot2\cdot3\cdot643\\[8pt]\end{aligned}$$

(view steps on how to factor 987654 and 123456. )

Step 2 : Put a box around factors that are common for all numbers:

$$\begin{aligned}987654 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{3}}\cdot97\cdot1697\\[8pt]123456 =& \color{blue}{\boxed{2}}\cdot2\cdot2\cdot2\cdot2\cdot2\cdot\color{red}{\boxed{3}}\cdot643\\[8pt]\end{aligned}$$

Step 3 : Multiply the boxed numbers together:

$$ GCD = 2\cdot3 = 6 $$

This solution can be visualized using a Venn diagram.

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

GCD Calculator