Question

ABCDE is a regular pentagon.

Answer 1

Part (A)

We have n = 5 sides, so the measure of any exterior angle of a regular pentagon is E = 360/n = 360/5 = 72

Each interior angle is therefore 180-E = 180-72 = 108 degrees.

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Alternatively, you could use the formula below with n = 5

i = interior angle

i = 180(n-2)/n

i = 180(5-2)/5

i = 180(3)/5

i = 540/5

i = 108

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Answer: 108 degrees

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Part (B)

We're told that CDF is 70 degrees. From part (A) we know that angle CDE is 108 degrees, so this must mean that angle EDF is 108-70 = 38 degrees.

Similarly, angle FEA is 85 degrees, which subtracts from angle AED to get 108 - 85 = 23 degrees. This is the measure of angle DEF.

So far we found

angle EDF = 38

angle DEF = 23

Let's call these x and y for shorthand. Let z be the missing third angle of triangle DFE.

So,

x+y+z = 180

38+23+z = 180

61+z = 180

x = 180-61

x = 119

Interior angle DFE is 119 degrees.

Note how 90 < x < 180 showing we have an obtuse angle. It is not a reflex angle because the statement 180 < x < 360 is false.

So we subtract the value of x from 360

360-x = 360-119 = 241

This is the reflex angle DFE

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Answer: 241 degrees