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python/数学课程/【翻折问题】角平分线.md

922 B

  • NF=x,而且AN+FD=NF,所以AD=BC=2x

  • 对角线常见的辅助线引法,有NH \perp BF 得到全等三角形\triangle ANB \cong \triangle NHB \therefore BH=AB=3,NH=AN

  • \because \triangle BFE \cong \triangle BEC \therefore BF=BC=2x\because BH=AB=3,\therefore HF=2x-3

相似三角形之反A形:\triangle FNH \sim FNA

\therefore \frac{FN}{FB}=\frac{NH}{AB}=\frac{FH}{AF}

\therefore \frac{x}{2x}=\frac{NH}{3}=\frac{2x-3}{AF}

\therefore \frac{1}{2}=\frac{NH}{3}=\frac{2x-3}{AF}

\therefore NH=\frac{3}{2} \therefore AN=\frac{3}{2}

\therefore \frac{1}{2}=\frac{2x-3}{\frac{3}{2}+x}

\therefore 4x-6=\frac{3}{2}+x 3x=\frac{15}{2}

\therefore x=\frac{5}{2}

\therefore BC=2x=5