Renan's error in adding the expressions is that he added incorrectly.
To clarify, let's go through the calculation:
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The original expression is \( (4.2x + 3) + (5.7x - 2) + (5 - 2.6x) \).
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Combining like terms for \(x\):
- \(4.2x + 5.7x - 2.6x = 4.2 + 5.7 - 2.6 = 7.3x\).
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Combining constant terms:
- \(3 - 2 + 5 = 6\).
Thus, the correct sum should be \( 7.3x + 6\), not \( 12.5x + 6\).
Hence, Renan made an error in the addition of the \(x\) terms.