The most frequent reason for this is because you are rounding the sides and angles which can, at times , lead to results that seem inaccurate. In these cases , in actuality , the calculator is really producing correct results. However, it is then rounding them for you- which leads to seemingly inaccurate results and possible error warnings . To see if that is your problem, set the rounding to maximum accuracy.
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Why is the calculator saying there's an error when there shouldn't be?