Feb 10 - Very Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R3C3 can only be <6>
R3C7 can only be <2>
R2C3 can only be <3>
R3C2 can only be <9>
R3C8 can only be <4>
R7C7 can only be <3>
R7C2 can only be <2>
R7C3 can only be <7>
R1C1 can only be <2>
R7C8 can only be <1>
R2C2 can only be <1>
R8C3 can only be <4>
R1C9 is the only square in row 1 that can be <3>
R1C5 is the only square in row 1 that can be <6>
R2C5 can only be <2>
R2C4 can only be <7>
R2C6 can only be <8>
R4C9 is the only square in row 4 that can be <1>
R6C2 is the only square in row 6 that can be <8>
R8C6 is the only square in row 8 that can be <7>
R9C4 is the only square in row 9 that can be <1>
R8C5 is the only square in column 5 that can be <9>
Squares R5C2 and R5C5 in row 5 form a simple naked pair. These 2 squares both contain the 2 possibilities <34>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R5C1 - removing <3> from <357> leaving <57>
R5C8 - removing <3> from <2357> leaving <257>
Squares R4C4 and R5C5 in block 5 form a simple naked pair. These 2 squares both contain the 2 possibilities <34>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R4C6 - removing <4> from <45> leaving <5>
R6C4 - removing <3> from <239> leaving <29>
Squares R6C4 and R6C6 in row 6 form a simple naked pair. These 2 squares both contain the 2 possibilities <29>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R6C8 - removing <2> from <2356> leaving <356>
R6C9 - removing <2> from <256> leaving <56>
Intersection of column 9 with block 6. The value <5> only appears in one or more of squares R4C9, R5C9 and R6C9 of column 9. These squares are the ones that intersect with block 6. Thus, the other (non-intersecting) squares of block 6 cannot contain this value.
R5C8 - removing <5> from <257> leaving <27>
R6C8 - removing <5> from <356> leaving <36>
Squares R2C7, R2C8, R8C7 and R8C8 form a Type-1 Unique Rectangle on <56>.
R8C8 - removing <56> from <256> leaving <2>
R8C4 can only be <3>
R5C8 can only be <7>
R9C9 can only be <6>
R9C1 can only be <3>
R6C9 can only be <5>
R8C7 can only be <5>
R5C1 can only be <5>
R5C9 can only be <2>
R8C2 can only be <6>
R4C4 can only be <4>
R9C5 can only be <4>
R2C7 can only be <6>
R6C1 can only be <6>
R9C6 can only be <2>
R5C5 can only be <3>
R6C6 can only be <9>
R2C8 can only be <5>
R1C4 can only be <9>
R5C2 can only be <4>
R6C8 can only be <3>
R4C1 can only be <7>
R4C2 can only be <3>
R6C4 can only be <2>
R1C6 can only be <4>
R4C8 can only be <6>
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