We are given a pentagon ABCDE with angles $A = 120^\circ$, $C = 110^\circ$, and $D = 135^\circ$. We are also given that $\angle AED = 4 \times \angle ABC$. We need to find the size of $\angle AED$.

GeometryPolygonsPentagonsAngle SumGeometric Proof
2025/4/1

1. Problem Description

We are given a pentagon ABCDE with angles A=120A = 120^\circ, C=110C = 110^\circ, and D=135D = 135^\circ. We are also given that AED=4×ABC\angle AED = 4 \times \angle ABC. We need to find the size of AED\angle AED.

2. Solution Steps

First, recall that the sum of the interior angles of a pentagon is given by the formula:
(n2)×180(n-2) \times 180^\circ
where nn is the number of sides. In this case, n=5n = 5, so the sum of the interior angles is:
(52)×180=3×180=540(5-2) \times 180^\circ = 3 \times 180^\circ = 540^\circ
We know that the sum of the angles in pentagon ABCDE is:
A+B+C+D+E=540\angle A + \angle B + \angle C + \angle D + \angle E = 540^\circ
Substituting the given values, we have:
120+ABC+110+135+AED=540120^\circ + \angle ABC + 110^\circ + 135^\circ + \angle AED = 540^\circ
365+ABC+AED=540365^\circ + \angle ABC + \angle AED = 540^\circ
We also know that AED=4×ABC\angle AED = 4 \times \angle ABC. Let x=ABCx = \angle ABC. Then AED=4x\angle AED = 4x. Substituting this into the equation above, we get:
365+x+4x=540365^\circ + x + 4x = 540^\circ
365+5x=540365^\circ + 5x = 540^\circ
5x=5403655x = 540^\circ - 365^\circ
5x=1755x = 175^\circ
x=1755x = \frac{175^\circ}{5}
x=35x = 35^\circ
Therefore, ABC=35\angle ABC = 35^\circ. Now we can find AED\angle AED:
AED=4×ABC=4×35=140\angle AED = 4 \times \angle ABC = 4 \times 35^\circ = 140^\circ

3. Final Answer

AED=140\angle AED = 140^\circ

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