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16A 17B 18B đây nha chị

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B

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16A

17B

18B

\(h'\left(x\right)=f'\left(x\right)-g'\left(x\right)=0\Rightarrow x=\left\{a;b;c\right\}\)

Ta thấy \(h'\left(x\right)>0\) trên \(\left(b;c\right)\) và \(h'\left(x\right)< 0\) trên \(\left(a;b\right)\)

\(\Rightarrow x=b\) là điểm cực tiểu trên \(\left[a;c\right]\) hay \(\min\limits_{\left[a;c\right]}h\left(x\right)=h\left(b\right)\)

 \(=>Qthu1=0,2.340000=68000J\)

\(=>Qthu2=2100.0,2.20=8400J\)

\(=>Qtoa=2.4200.25=210000J\)

\(=>Qthu1+Qthu2< Qtoa\)=>đá nóng chảy hoàn toàn

\(=>0,2.2100.20+0,2.340000+0,2.4200.tcb=2.4200\left(25-tcb\right)\)

\(=>tcb=14,5^oC\)