Compared to Naked Pair Technique, Hidden Pair Technique is much more difficult to use. Unlike Naked Pairs having the identical two candidates in both two cells, cells of a Hidden Pair may contain a lot more candidates. To understand this, let's start with an example:
In Row A, numbers 3 and 6 occur at cells [A4] and [A8] only. That is, no other cells in this row can be placed with 3 or 6. Suppose if [A4] = 3, then [A8] must be 6; or if [A4] = 6, then [A8] must be 3. There is no other circumstance. Thus, we can eliminate all candidates other than 3 and 6 from cells [A4] and [A8]. After the elimination, we can use Hidden Single Technique against cell [A7] since it now has the only 9 in row A.
Here is another example:
In Column 1, numbers 2 and 9 are found only in candidates at cells [G1] and [I1]. Therefore we can remove all other numbers from these two cells safely.
Commonly, we can find Hidden Pairs in blocks as well:
Candidates 2 and 8 are only located in two highlighted cells of Block at [D4] and therefore form a Hidden Pair. All candidates except 2 and 8 can then be safely excluded from these two cells.
As a summary, we can apply Hidden Pair Techniqueonly if two given numbers happen to be both occur and only occur in two cells of the same unit. When this happens, we can eliminate all other candidates from these two cells.
Hidden Pairs are not as easy to find as Naked Pairs. In addition, Naked Pair Technique is applied against the cells in a unit other than those with Naked Pairs, while Hidden Pair Technique affects only the cells containing Hidden Pairs. But both techniques help reduce the number of candidates and therefore create chances to apply other elimination techniques. It is even interesting to find that applying Hidden Pair Technique will cause a Hidden Pair to become a Naked Pair.
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