The Encyclopedia of Geometry (0072)


Problem

In a triangle $ABC$, let $∠B<∠C$ and draw the perpendiculars $BD$ and $CE$ from $B$ and $C$ to their opposite sides, respectively. Then, we have

$$CE>BD.$$  


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Solution

If we extend $BD$ and take a point $B’$ such that $BD=DB’$, and extend $CE$ and take a point $C’$ such that $CE=EC’$, then $△BCB’$ is an isosceles triangle with $C$ as the vertex, and $△CBC’$ is an isosceles triangle with $B$ as the vertex, and

$$BC’=BC=CB’.$$

Moreover,

$$∠CBC’=2∠B \qquad and \qquad ∠BCB’=2∠C.$$

Since we assume $∠B<∠C$,

$$∠CBC’>∠BCB’.$$

Since $△BCB’$ and $△CBC’$ share the side $BC$, and $CB’=BC’$ and $∠CBC’>∠BCB’$,

$$CC’>BB’.$$

However,

$$CE=\frac{1}{2} CC’ \qquad and \qquad BD=\frac{1}{2} BB’,$$

$$∴ \ CE>BD.$$

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Reference

Teiichiro Sasabe (1976) The Encyclopedia of Geometry (2nd edition), Seikyo-Shinsha, p.18