The number where the middle three circles intersect 1 has been used for three times, the number where every two circles intersect has been used for three times, and the number which exists alone around has been used for three times 1.

Fill in 3 in the middle, 1, 4, 6 in the order of crossing each other, and 2, 5, 7 in the periphery;

The combinations are (1, 3, 4 and 7), (1, 3, 5 and 6) and (2, 3, 4 and 6) respectively.

Find a routine method:

Find patterns and fill in numbers, or find patterns by figures. At first, everyone found patterns through some comparisons. Maybe some series of three numbers have a "pattern", but it is not certain, it can only be regarded as a guess. Generally, it needs more than three, including the combination of before and after control to confirm the law.

Whether looking for laws in sequence or in graphics, we need keen observation. In particular, some laws are deeply hidden and need to be bold and cautious to discover. Finally, after filling it out, it is necessary to check whether the rules found before and after are correct, so that it will not be in vain.