Find, in degrees, the sum of the marked angles.
Remark : The above diagram consists of 19 points, A 0 , A 1 , … , A 1 8 , there are 19 line segments of form A i A i + 7 , where the subscripts read modulo 19.
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Or you can just apply the formula: 1 8 0 ∘ n − 3 6 0 ∘ k , where n = 1 9 , k = 7 .
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An 8 lines or 8 vertices could have 3 6 0 ∘ while a 7 lines or 7 vertices could have only 1 8 0 ∘ . 5 times of 1 8 0 ∘ is a good question. A i A i + 7 gives k = 7 is not easy to determine to me.
Let the number of points be n , the angles equal to x i ∘ ( i = 1 , … , n ), and the number of times the figure turns around m .
In this case, n = 1 9 and m = 7 .
If you trace the figure, then at every angle x ∘ you turn ( 1 8 0 − x ) ∘ . Thus the total angle over which you turn is 1 8 0 n − i = 1 ∑ n x i = 3 6 0 m . Solving this gives i = 1 ∑ n x i = 1 8 0 n − 3 6 0 m . In this case, i = 1 ∑ n x i = 1 8 0 × 1 9 − 3 6 0 × 7 = 1 8 0 × ( 1 9 − 2 × 7 ) = 1 8 0 × 5 = 9 0 0 ∘ .
I don't think I have the same good sights as Chan Lye Lee. Here is my way:
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Sum of proximity gave 902. But I know it must be INT(902/ 180)*180, therefore it is equals to 900.
Answer: 9 0 0
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!
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Yes. Otherwise there shall be no other possibility to me. The reason is features of whether 1 8 0 ∘ or 3 6 0 ∘ 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.
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That is fine. As long as we are enjoying the problem solving.
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@Chan Lye Lee – We should enjoy problem solving.
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Each of the '5-stars' and '7-star' is of angle-sum 180 degrees, which make the desired answer to be 900 degrees.