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Geometry Level 4

What is the measurement of the blue angle in degrees?


This is part of the series: " It's easy, believe me! "


The answer is 135.

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1 solution

At first we can find out that A B F = 6 0 o \angle ABF =60^{o} in many ways, and this means that A B A C \overline{AB} \cong \overline{AC} (As result of its similarity with any half equilateral triangle). Then, we know that A B C \bigtriangleup ABC is isosceles D B C = 1 5 o \Rightarrow \angle DBC=15^{o} . As the line segment F C \overline{FC} is an altitude and at once a median, we can notice that D B C \bigtriangleup DBC is, too, isosceles, so we have: ( B C D C E D C = 7 5 o ) E D C A B C D E C = 3 0 o (\overline{BC} \cong \overline{DC} \wedge \angle EDC=75^{o}) \Rightarrow \bigtriangleup EDC \cong \bigtriangleup ABC \Rightarrow \angle DEC=30^{o} And, by using the exterior angle theorem in tiangles E D C \bigtriangleup EDC and D B C \bigtriangleup DBC we find: x = 9 0 o + 4 5 o = 13 5 o x=90^{o}+45^{o}=135^{o}

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