Measure of D = ? \angle D =?

Geometry Level 2

In the diagram above,

A B AB is parallel to E H EH , and B D BD is parallel to F H FH .

Also, A B = B C AB=BC and E F = F H EF=FH .

If E G C = 7 0 \angle EGC= 70^\circ , then D = ? \angle D= ?

8 5 85^\circ 8 0 80^\circ 9 0 90^\circ 7 5 75^\circ 6 5 65^\circ

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2 solutions

Marta Reece
Jun 25, 2017

E G C = 7 0 C A B = 7 0 \angle EGC=70^\circ\implies\angle CAB=70^\circ because A B AB and E H EH are parallel.

A B = B C A C B = 7 0 AB=BC\implies \angle ACB=70^\circ as well, and therefore also D C G = 7 0 \angle DCG=70^\circ

Let C D F = x \angle CDF=x . Then because B D BD is parallel to F H FH the E F H = 18 0 x \angle EFH=180^\circ-x .

F E H = 1 2 ( 180 E F H ) = x 2 \angle FEH=\frac12(180-\angle EFH)=\frac x2

D E G = 18 0 E F H = 18 0 x 2 \angle DEG=180^\circ-\angle EFH=180^\circ-\frac x2

The sum of the angles in rectangle D E G C DEGC is

36 0 = x + 7 0 + 7 0 + 18 0 x 2 360^\circ=x+70^\circ+70^\circ+180^\circ-\frac x2

From that x = 8 0 x=\boxed{80^\circ}

Thank you.

Hana Wehbi - 3 years, 11 months ago

DEGC isn't a rectangle.

Vaibhav Priyadarshi - 2 years, 11 months ago
Áron Bán-Szabó
Jun 25, 2017

E G C = 70 ° \angle EGC=70° , so ( A B AB and E H EH are parallel) C A B = 70 ° \angle CAB=70° .

A B = B C AB=BC , so A C B = 70 ° = D C G \angle ACB=70°=\angle DCG

If M M is the midpoint of B D BD and E H EH (because they can't be parallel), then C M G = 180 ° 2 70 ° = 40 ° \angle CMG=180°-2*70°=40° . Since B D BD and F H FH are parallel, E H F = 40 ° \angle EHF=40° . So C D E = 360 ° D C G C G E G E D = 360 ° 70 ° 70 ° 140 ° = 80 ° \angle CDE=360°-\angle DCG-\angle CGE-\angle GED=360°-70°-70°-140°=\boxed{80°}

Thank you.

Hana Wehbi - 3 years, 11 months ago

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