Find the value of this angle

Geometry Level 3

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 60^\circ and 7 0 , 70^\circ, find the measure of the orange angle (in degrees).


Inspired by Epsilon07


The answer is 95.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Host3H .
Jul 3, 2017

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.

The final vertex needs not lie on the line joining the midpoint of the sides.

Vishwash Kumar ΓΞΩ - 3 years, 11 months ago

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 50^{\circ} ] one, if we complete this triangle. The answer is 7 0 + 2 5 = 9 5 70^{\circ} + 25^{\circ} = 95^{\circ} Q E D \boxed{QED} .

WLOG Let A(-10,0), B(10,0), so AB be the bottom line, angle CAB=70 and angle ABD= 60.
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 ( S i n 70 + S i n 60 ) / 2 x ( C o s 70 c o s 60 ) / 2 = 8 5 o . B M N = 180 85 = 9 5 o . \dfrac{x(Sin70+Sin60)/2}{-x(Cos70 - cos60 )/2}= 85^o.\\ \therefore~ BMN=180-85=\Large~~\color{#D61F06}{95^o} .



0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...