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The sum of a number and additive inverse is zero.

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$\"\"$ (Subtract 12.1 from each side)

$\"\"$ (Subtract: $\"\"$ ) $\"\"$

The product of a number and multiplicative inverse is one.

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$\"\"$ (Divide each side by $\"\"$ )

$\"\"$ (Cancel common terms)

$\"\"$ $\"\"$ $\"\"$

$\"\"$ .