A B C , D is a point such that D A = 3 , D B = 4 , and D C = 5 .
In an equilateral triangleWhat is the measurement of A D B in degrees?
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Draw a point E such that ADE is an equilateral triangle (look at the picture). Therefore AE = AD = ED = 3 and E A D = E D A = 60 (degrees).
Since ABC is an equilateral triangle, B A C = 60 (degree) and AC = AB. Therefore, B A D + C A D = B A E + B A D . Hence, C A D = B A E .
Now look at the triangles AEB and ADC. They must be equal to each other because AE=AD; C A D = B A E 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 = 90 (degrees)
In conclusion, A D B = A D E + E D B = 90 + 60 = 1 5 0 (degrees).
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(I labeled the points wrong so bear with me).
Rotate △ A B C about point A by 60 degrees to form a second triangle, as shown. Then P C = P ′ C ′ = 3 , P B = P ′ C = 5 , and A P = A P ′ = 4 . Because P ′ is the image of point P after being rotated, P A P ′ = 6 0 ∘ . Also notice that A P = A P ′ , so △ A P P ′ must be equilateral. Thus, angle A P P ′ is 6 0 ∘ . Then, because C P = 3 , P P ′ = 4 , and C P ′ = 5 , angle C P P ′ is right. Finally, the angle C P A = C P P ′ + P ′ P A = 9 0 ∘ + 6 0 ∘ = 1 5 0 ∘ .