Samuel has decided to cut a quadrilateral into 2 parts such that the 2 resulting quadrilaterals have identical side lengths, as shown in the diagram. Given the 2 angles 6 0 ∘ and 7 0 ∘ , find the measure of the orange angle (in degrees).
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The final vertex needs not lie on the line joining the midpoint of the sides.
Complete the triangle. See the properties of this line (segment ) her on CTK. . So, clearly the given line is parallel to the bisector of the apex angle [ 5 0 ∘ ] one, if we complete this triangle. The answer is 7 0 ∘ + 2 5 ∘ = 9 5 ∘ Q E D .
M(0,0) midpoint of AB, N midpoint of CD. N' projection of N on AB.
AC=BD=x. So C(-10+xCos70, xSin70) and D(10 - xcos60, xSin60).
So N( {-10+xCos70 + 10 - xcos60 }/2, x{Sin70+Sin60}/2 ) ...so N'( x{Cos70 - cos60 }/2, 0).
ArcTanAMN= ArcTanN'MN= ArcTanNN'/N'M= ArcTan
−
x
(
C
o
s
7
0
−
c
o
s
6
0
)
/
2
x
(
S
i
n
7
0
+
S
i
n
6
0
)
/
2
=
8
5
o
.
∴
B
M
N
=
1
8
0
−
8
5
=
9
5
o
.
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My solution goes like this:
Extend the lengths of the non-bisected sides until you have a triangle instead of a quadrilateral. It already has angles of 70 and 60 degrees, so the final angle is 180 - 70 - 60 = 50 degrees.
The final vertex is also on the bisecting line which results in the bisecting line splitting the new angle in half (2 x 25 degrees), thus creating two triangles. Solving for the one in which the unknown value lies, we find that 180 - 60 - 25 = 95 degrees.