We'll let you guess.

Geometry Level 3

In an equilateral triangle A B C ABC , D D is a point such that D A = 3 DA = 3 , D B = 4 DB = 4 , and D C = 5 DC = 5 .

What is the measurement of A D B ^ \widehat{ADB} in degrees?


The answer is 150.

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2 solutions

Brian Miyatake
Jul 1, 2019

(I labeled the points wrong so bear with me).

Rotate A B C \triangle ABC about point A A by 60 degrees to form a second triangle, as shown. Then P C = P C = 3 PC=P'C'=3 , P B = P C = 5 PB=P'C=5 , and A P = A P = 4 AP=AP'=4 . Because P P' is the image of point P P after being rotated, P A P = 6 0 PAP'=60^{\circ} . Also notice that A P = A P AP=AP' , so A P P \triangle APP' must be equilateral. Thus, angle A P P APP' is 6 0 60^{\circ} . Then, because C P = 3 CP=3 , P P = 4 PP'=4 , and C P = 5 CP'=5 , angle C P P CPP' is right. Finally, the angle C P A = C P P + P P A = 9 0 + 6 0 = 15 0 CPA=CPP'+P'PA=90^{\circ}+60^{\circ}=150^{\circ} .

Tin Le
Apr 7, 2019

Draw a point E such that ADE is an equilateral triangle (look at the picture). Therefore AE = AD = ED = 3 and E A D ^ \widehat{EAD} = E D A ^ \widehat{EDA} = 60 (degrees).

Since ABC is an equilateral triangle, B A C ^ \widehat{BAC} = 60 (degree) and AC = AB. Therefore, B A D ^ + C A D ^ \widehat{BAD}+\widehat{CAD} = B A E ^ + B A D ^ \widehat{BAE}+\widehat{BAD} . Hence, C A D ^ = B A E ^ \widehat{CAD}=\widehat{BAE} .

Now look at the triangles AEB and ADC. They must be equal to each other because AE=AD; C A D ^ = B A E ^ \widehat{CAD}=\widehat{BAE} and AC = AB.

Hence EB=CD=5.

The triangle DEB has DE = 3; DB = 4; EB = 5. According to the Pythagorean theorem, DEB must be a right triangle with E D B ^ \widehat{EDB} = 90 (degrees)

In conclusion, A D B ^ \widehat{ADB} = A D E ^ + E D B ^ \widehat{ADE}+\widehat{EDB} = 90 + 60 = 150 \boxed{150} (degrees).

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