Consider an equilateral triangle $\triangle ABC$. The points $K$ and $L$ divide the leg $BC$ into three equal parts, the point $M$ divides the leg $AC$ in the ratio $1:2$, counting from the vertex $A$. Prove that $\angle AKM+\angle ALM=30^{\circ}$. Proposed by V. Proizvolov
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Tags: ratio, geometry, trapezoid
18.05.2014 07:09
We have that each of $\angle CAK, \angle KAC$, and $\angle BAC$ are $20^{\circ}$. We also know that $ABLM$ is an isosceles trapezoid with where $\angle BCM$ is $120^{\circ}$. In $\triangle ABL$, we know that $\angle BLA$ is $100^{\circ}$ so then by subtraction, we know that $\angle ALM$ is $20^{\circ}$. Also, $\triangle CMK$ is a $30-60-90$ triangle where $\angle MKC$ is right. Thus we know that $\angle MKL$ is also right. Since $\triangle AKL$ is isosceles with $\angle KAL$ being $20^{\circ}$, $\angle AKL$ is $80^{\circ}$ and it follows that $\angle AKM=10^{\circ}$. Ergo, $ \angle AKM+\angle ALM= 10+20=\boxed{30^{\circ}}$
18.05.2014 08:22
See attached; $\angle AKM=\angle BAK=\angle LAC$, hence $\angle AKM+\angle ALM=\angle LAM+\angle ALM=\angle LMC=30^\circ$. Best regards, sunken rock
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