Câu 25:
\(p=\dfrac{a+b+c}{2}=\dfrac{39+25+16}{2}=\dfrac{80}{2}=40.\)
Áp dụng công thức Herong: \(S=\sqrt{p\left(p-a\right)\left(p-b\right)\left(p-c\right)}\) ta có:
\(S=\sqrt{40\left(40-39\right)\left(40-25\right)\left(40-16\right)}=120\) (đvdt).
Ta có: \(S=\dfrac{abc}{4R}.\) \(\Rightarrow120=\dfrac{39.25.16}{4R}.\\ \Leftrightarrow120=\dfrac{3900}{R}.\\ \Leftrightarrow R=32,5.\)
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