From Twinkle Twinkle Little Star

Geometry Level 5

Find, in degrees, the sum of the marked angles.

Remark : The above diagram consists of 19 points, A 0 , A 1 , , A 18 A_0, A_1, \ldots, A_{18} , there are 19 line segments of form A i A i + 7 A_{i}A_{i+7} , where the subscripts read modulo 19.


The answer is 900.

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3 solutions

Chan Lye Lee
Nov 17, 2015

Each of the '5-stars' and '7-star' is of angle-sum 180 degrees, which make the desired answer to be 900 degrees.

Or you can just apply the formula: 18 0 n 36 0 k 180^\circ n - 360^\circ k , where n = 19 , k = 7 n = 19, k = 7 .

Pi Han Goh - 5 years, 6 months ago

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An 8 lines or 8 vertices could have 36 0 360^\circ while a 7 lines or 7 vertices could have only 18 0 . 180^\circ. 5 times of 18 0 180^\circ is a good question. A i A i + 7 A_i A_{i+7} gives k = 7 is not easy to determine to me.

Lu Chee Ket - 5 years, 6 months ago

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I have no idea what you're talking about.

Pi Han Goh - 5 years, 6 months ago
Arjen Vreugdenhil
Nov 21, 2015

Let the number of points be n n , the angles equal to x i x_i^\circ ( i = 1 , , n i = 1,\dots,n ), and the number of times the figure turns around m m .

In this case, n = 19 n = 19 and m = 7 m = 7 .

If you trace the figure, then at every angle x x^\circ you turn ( 180 x ) (180-x)^\circ . Thus the total angle over which you turn is 180 n i = 1 n x i = 360 m . 180n - \sum_{i=1}^n x_i = 360m. Solving this gives i = 1 n x i = 180 n 360 m . \sum_{i=1}^n x_i = 180n-360m. In this case, i = 1 n x i = 180 × 19 360 × 7 = 180 × ( 19 2 × 7 ) = 180 × 5 = 900 . \sum_{i=1}^n x_i = 180\times 19-360\times 7 = 180\times(19-2\times 7) = 180\times 5 = \boxed{900}^\circ.

Lu Chee Ket
Nov 19, 2015

I don't think I have the same good sights as Chan Lye Lee. Here is my way:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
A0 =    48
A1 =    48
A2 =    49
A3 =    50
A4 =    51
A5 =    51
A6 =    50
A7 =    50
A8 =    44
A9 =    41
A10 =   50
A11 =   50
A12 =   49
A13 =   50
A14 =   51
A15 =   39
A16 =   40
A17 =   44
A18 =   47

Sum of proximity gave 902. But I know it must be INT(902/ 180)*180, therefore it is equals to 900.

Answer: 900 \boxed{900}

Do you measure it by hand? It is not the intention, but you can always come with any method. Good try and good answer anyway!

Chan Lye Lee - 5 years, 6 months ago

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Yes. Otherwise there shall be no other possibility to me. The reason is features of whether 18 0 180^\circ or 36 0 360^\circ is not so direct and simple to add on without a careful study. Seems to be way dependent. I was taking no risk for a mistake.

Lu Chee Ket - 5 years, 6 months ago

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That is fine. As long as we are enjoying the problem solving.

Chan Lye Lee - 5 years, 6 months ago

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@Chan Lye Lee We should enjoy problem solving.

Lu Chee Ket - 5 years, 6 months ago

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