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# Solving Techniques 17 Hidden Unique Rect's.

This is a method of finding hidden unique rectangles.
Below, the [6] in the [156] is X’d out. It can be eliminated as a candidate.

If the upper left blue [156] cell is a [1], then [6] can’t be a candidate for this cell.

A problem arises when the blue [156] is any number other than [1]. The yellow parts become candidates for [1].

Then the lower right blue cell would be a [6], since it is a [16]. In this case, too, if the upper left, blue cell is a [6]…

we eventually arrive at the following diagram.

But lets step back. The four blue cells create a unique rectangle with [16][16], leading to multiple solutions. Therefore, in this case, the blue cell to the upper left is a [5] and not a [6].

Hence, this [6] can be removed as a candidate. Otherwise, we are left with a Sudoku problem which doesn’t work.

## Hidden Unique Rect's. 2

Below is also a hidden unique rectangle. The X’d out [6] can be removed as a candidate from D3. (The 8s in the same row form a strong link)

If the [68] in the upper left B1 is an [8], then B3 becomes a [6], so the X’d out [6] in D3 can’t be entered there.

If the upper left B1’s [68] is a [6], we have a problem. In this case, there are only two [8]’s in the same row, so even if there are many candidates for D1, it will be an [8]. Furthermore, B3 will also be an [8]. If D3 is a [6], we have a unique triangle where [6] and [8] are interchangeable.

It will hence, not work as a Sudoku problem, and [6] can be removed as a candidate.

reference: sudokuwiki.org

## Names of cells in Sudoku

 A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 B3 B4 B5 B6 B7 B8 B9 C1 C2 C3 C4 C5 C6 C7 C8 C9 D1 D2 D3 D4 D5 D6 D7 D8 D9 E1 E2 E3 E4 E5 E6 E7 E8 E9 F1 F2 F3 F4 F5 F6 F7 F8 F9 G1 G2 G3 G4 G5 G6 G7 G8 G9 H1 H2 H3 H4 H5 H6 H7 H8 H9 I1 I2 I3 I4 I5 I6 I7 I8 I9