Feb 18 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R3C8 is the only square in row 3 that can be <7>
R9C3 is the only square in row 9 that can be <8>
R5C1 is the only square in column 1 that can be <2>
R5C8 is the only square in column 8 that can be <5>
R2C9 is the only square in column 9 that can be <8>
Squares R4C3 and R4C6 in row 4 form a simple naked pair. These 2 squares both contain the 2 possibilities <35>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R4C4 - removing <35> from <3459> leaving <49>
R4C5 - removing <35> from <23459> leaving <249>
R4C7 - removing <3> from <239> leaving <29>
Squares R4C6 and R6C4 in block 5 form a simple naked pair. These 2 squares both contain the 2 possibilities <35>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R5C4 - removing <3> from <389> leaving <89>
R5C5 - removing <3> from <36789> leaving <6789>
R5C6 - removing <3> from <367> leaving <67>
R6C5 - removing <35> from <12357> leaving <127>
R6C6 - removing <35> from <1357> leaving <17>
Intersection of column 8 with block 9. The values <12> only appears in one or more of squares R7C8, R8C8 and R9C8 of column 8. These squares are the ones that intersect with block 9. Thus, the other (non-intersecting) squares of block 9 cannot contain these values.
R9C7 - removing <2> from <2679> leaving <679>
Intersection of block 4 with column 3. The value <5> only appears in one or more of squares R4C3, R5C3 and R6C3 of block 4. These squares are the ones that intersect with column 3. Thus, the other (non-intersecting) squares of column 3 cannot contain this value.
R1C3 - removing <5> from <1356> leaving <136>
R2C3 - removing <5> from <1356> leaving <136>
R8C3 - removing <5> from <157> leaving <17>
Intersection of block 8 with column 5. The value <3> only appears in one or more of squares R7C5, R8C5 and R9C5 of block 8. These squares are the ones that intersect with column 5. Thus, the other (non-intersecting) squares of column 5 cannot contain this value.
R1C5 - removing <3> from <13458> leaving <1458>
R2C5 - removing <3> from <1345> leaving <145>
R3C5 - removing <3> from <34> leaving <4>
R3C2 can only be <3>
R5C2 can only be <7>
R5C6 can only be <6>
R7C2 can only be <2>
R7C8 can only be <3>
R7C5 can only be <7>
R8C9 can only be <9>
R5C9 can only be <3>
R6C7 can only be <2>
R4C7 can only be <9>
R6C5 can only be <1>
R9C6 can only be <5>
R9C2 can only be <1>
R9C4 can only be <9>
R4C6 can only be <3>
R4C3 can only be <5>
R1C6 can only be <1>
R6C4 can only be <5>
R4C4 can only be <4>
R4C5 can only be <2>
R6C3 can only be <3>
R6C6 can only be <7>
R2C5 can only be <5>
R8C3 can only be <7>
R9C5 can only be <6>
R5C4 can only be <8>
R9C7 can only be <7>
R9C8 can only be <2>
R8C5 can only be <3>
R8C7 can only be <6>
R1C3 can only be <6>
R2C1 can only be <4>
R1C5 can only be <8>
R5C5 can only be <9>
R1C4 can only be <3>
R8C8 can only be <1>
R1C8 can only be <9>
R2C3 can only be <1>
R1C7 can only be <4>
R2C7 can only be <3>
R1C2 can only be <5>
R2C8 can only be <6>
R2C2 can only be <9>
R8C1 can only be <5>
R8C2 can only be <4>
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