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# In the figure, ABCD and BEFG are squares, and BCE is an equilateral triangle. What is the number of degrees in angle GCE ?

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In the figure, ABCD and BEFG are squares, and BCE  is an equilateral triangle. What is the number of degrees in angle GCE ?

tertre  Apr 18, 2017
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#1
+79819
+2

Draw a circle with B  as a center and BA as a radius....

This circle will pass through the points  A, C, E  and G

And angle EBG  will  be a central angle that will intercept the minor arc  EG

And the measure of this angle = 90°

And angle  GCE  will be an inscribed angle  intercepting the same arc as  angle EBG

And an inscribed angle intercepting the same arc as a central angle will have 1/2 the measure of that angle......so.....  angle GCE  =  (1/2) (90°)  =  45°

CPhill  Apr 18, 2017
edited by CPhill  Apr 18, 2017
#2
+5552
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Wow, that's really good!

I would not have thought to draw a circle !

Here is a picture, I thought I might as well share it:

hectictar  Apr 18, 2017
#3
+79819
+1

CPhill  Apr 18, 2017
#4
+79819
+2

Not to beat a dead horse, but.....here's a way easier solution based on hectictar's pic.....no circle drawing is actually required!!!!

Note that triangle CBG is isosceles with angle  CBG  = 150

Since CB  = BG.....so...  angle BCG  =  [ 180 - 150] / 2   =  15

But angle BCE  =  60  =   angle BCG + angle GCE

So

60  = 15  + angle GCE

45  =  angle GCE

CPhill  Apr 18, 2017
edited by CPhill  Apr 18, 2017
#5
+5552
+2

Oh yeah! I see that now too..

I really like the circle solution though.. it's very clever!

hectictar  Apr 18, 2017
#6
+18777
+1

In the figure, ABCD and BEFG are squares, and BCE  is an equilateral triangle.

What is the number of degrees in angle GCE ?

Center of a circle in B with radius r=BG=BE=BC

Central Angle Theorem:

The angle at the centre is twice the angle at the circumference.

$$\beta = \frac{\alpha}{2}=\frac{90^{\circ}}{2}=45^{\circ}$$

heureka  Apr 18, 2017
edited by heureka  Apr 18, 2017

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