Bài làm
Xét tam giác ABC có:
\(\widehat{BAC}+\widehat{ABC}+\widehat{ACB}=180^0\)
=> \(\widehat{BAC}=180^0-\widehat{ABC}-\widehat{ACB}\)
=> \(\widehat{BAC}=180^0-\widehat{ABC}-\widehat{ACB}\)
Mà \(\widehat{BAC}=\widehat{ABC}-\widehat{ACB}\)
=> \(180^0-\widehat{ABC}-\widehat{ACB}=\widehat{ABC}-\widehat{ACB}\)
=> \(180^0-\widehat{ABC}-\widehat{ABC}-\widehat{ACB}+\widehat{ACB}=0\)
=> \(180^0-2\widehat{ABC}=0\)
=> \(2\widehat{ABC}=180^0\)
=> \(\widehat{ABC}=180^0:2=90^0\)
Ai làm được nốt không?
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\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)
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