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Evaluating 20z20 - z for z=12z = 12 and z=12z = -12

Evaluate the expression 20z20 - z for two different values of zz:

(a) When z=12z = 12: Replace zz with 1212 to obtain 201220 - 12. Subtract: 2012=820 - 12 = 8.

(b) When z=12z = -12: Replace zz with 12-12 to obtain 20(12)20 - (-12). Subtracting a negative is equivalent to adding the corresponding positive: 20(12)=20+12=3220 - (-12) = 20 + 12 = 32.

Comparing the two results reveals an important pattern: substituting a positive value and its opposite into the same subtraction expression produces very different outcomes (88 versus 3232). When the variable being subtracted is negative, the double negative converts the subtraction into addition, which increases the overall value rather than decreasing it.

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Updated 2026-04-21

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