Japan “Sangaku” Research Institute

Problem In a triangle $ABC$, let $X$ be the point where the bisector of $∠C$ intersects the side $AB$. Let $Y$ and $Z$ be the points where a straight line drawn parallel to the side $AC$ through $X$ intersects the side $BC$ and the bisector of the exterior angle of $∠C$, respectively. Then $$XY=YZ.$$ $$ $$…

Problem In a triangle $ABC$ such that $∠B=3∠C$, if $BD$ is the perpendicular drawn from $B$ to the bisector of $∠A$, then $$BD=\frac{1}{2} (AC-AB).$$ $$ $$ $$ $$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ Solution Let $E$ be the point where the…

Problem Let $D$ be the midpoint of the side $BC$ of a triangle $ABC$, and let $E$ and $F$ be the points where the bisectors of $∠ADB$ and $∠ADC$ intersect the sides $AB$ and $AC$, respectively. Then $$EF<BE+CF.$$ $$ $$ $$ $$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$…

Problem In a triangle $ABC$, if we take any point $P$ on the bisector of $∠A$, we have $$|AB-AC|>|PB-PC|.$$ However, let $AB≠AC$. $$ $$ $$ $$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ Solution $(1)$ Let $AB>AC$. If we take a point $D$…

Problem First, create a new triangle $A’B’C’$ by connecting the intersections of the bisectors of each exterior angle of the original triangle $ABC$. Next, create a triangle $A^”B^”C^”$ by connecting the intersections of the bisectors of each exterior angle of the triangle $A’B’C’$. If you create new triangles one after another in this way, the…

Problem In a triangle $ABC$, if $∠B>∠C$, then the bisector $BD$ of $∠B$ is smaller than the bisector $CE$ of $∠C$. $$ $$ $$ $$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ Solution Since $∠B>∠C$, $$∠ABD>∠ACE.$$ Now, if we take $∠DBF$ equal to…

P roblem In a triangle $ABC$, if the intersection of the bisectors of $∠B$ and $∠C$ is $O$, then $$∠BOC=90°+\frac{1}{2}∠A.$$ Also, if the intersection of the bisectors of the exterior angles at $B$ and $C$ is $O’$, then $$∠BO’ C=90°-\frac{1}{2}∠A.$$ $$ $$ $$ $$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$…

P roblem Is the statement “The bisector of the apex angle of a triangle and the perpendicular bisector of the opposite side intersect at a point outside the triangle” correct? $$ $$ $$ $$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ Solution The…

P roblem In a triangle $ABC$, the angle formed by the intersection $E$ of the bisectors of $∠B$ and the exterior angle of $∠C$ is equal to half of $∠A$. $$ $$ $$ $$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $\downarrow$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ Solution Let…