[Math Revolution GMAT math practice question]
What is the measure of each interior angle of a regular decagon?
A. 72°
B. 108°
C. 120°
D. 135°
E. 144°
What is the measure of each interior angle of a regular deca
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- Max@Math Revolution
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$$\sum\nolimits_N {\,\, = \,\,\,\left( {N - 2} \right) \cdot 180\,\,\,\,\,\, \Rightarrow \,\,\,\,\,} \sum\nolimits_{10} {\,\, = \,\,\,\left( {10 - 2} \right) \cdot 180 = 8 \cdot 180} \,\,\,\,\,\,\,\left[ {{\text{degrees}}} \right]$$Max@Math Revolution wrote:[Math Revolution GMAT math practice question]
What is the measure of each interior angle of a regular decagon?
A. 72°
B. 108°
C. 120°
D. 135°
E. 144°
$$?\,\,\,\mathop = \limits^{{\text{regular}}} \,\,\,\frac{{\sum\nolimits_{10} {} }}{{10}} = \underleftrightarrow {8 \cdot \left( {20 - 2} \right) = 160 - 16} = 144\,\,\,\left[ {{\text{degrees}}} \right]$$
We follow the notations and rationale taught in the GMATH method.
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Useful rule: the sum of the angles in an n-sided polygon = (n - 2)(180º)Max@Math Revolution wrote:[Math Revolution GMAT math practice question]
What is the measure of each interior angle of a regular decagon?
A. 72°
B. 108°
C. 120°
D. 135°
E. 144°
So, for example, the sum of the angles in an 11-sided polygon = (11 - 2)(180º)
= (9)(180º)
= 1620º
What is the measure of each interior angle of a regular decagon?
A decagon has 10 sides.
The sum of the angles in an 10-sided polygon = (10 - 2)(180º)
= (8)(180º)
= 1440º
In a REGULAR polygon, all angles are equal
So, in a decagon, each angle = 1440º/10 = 144º
Answer: E
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- Max@Math Revolution
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=>
The sum of all interior angles of n-gon is (n-2)*180°.
A decagon is a 10-gon.
The sum of all interior angles is (10-2)* 180° = 8*180°.
And each interior angle of a regular decagon has measure 8*180° / 10 = 8*18° = 144°.
Therefore, the answer is E.
Answer: E
The sum of all interior angles of n-gon is (n-2)*180°.
A decagon is a 10-gon.
The sum of all interior angles is (10-2)* 180° = 8*180°.
And each interior angle of a regular decagon has measure 8*180° / 10 = 8*18° = 144°.
Therefore, the answer is E.
Answer: E
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A decagon is a polygon with 10 sides and thus 10 angles. We use the formula for the total number of degrees in an n-gon as n(180 - 2).Max@Math Revolution wrote:[Math Revolution GMAT math practice question]
What is the measure of each interior angle of a regular decagon?
A. 72°
B. 108°
C. 120°
D. 135°
E. 144°
Thus, the sum of the measures of all 10 angles is 180(10 - 2) = 180(8) = 1440 degrees. A regular decagon is one that has all equal sides and equal angles. Thus, the measure of each angle of a regular decagon is 1440/10 = 144 degrees.
Alternate Solution:
We can also use the fact that the exterior angles of all polygons add up to 360 degrees. Since a regular decagon has all equal sides and equal interior angles, all the exterior angles are also equal and thus, an exterior angle of a regular decagon is 360/10 = 36 degrees. If an exterior angle is 36 degrees, then an interior angle is 180 - 36 = 144 degrees.
Answer: E
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