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In a triangle with integer side lengths, one side is three times as long as a second side, and the length of third side is 17. What is the difference between the greatest and smallest possible perimeter of the triangle?

A. 15
B. 12
C. 25
D. 15
E. 13[/quote]

Three sides of a triangle are a,b,and c.
First side is a
Second side is b
Third side is c
a = 3b, b= b,c=17 Given

a -b < c < a+b by side property from triangle
2b < 17 < 4b
Perimeter of a triangle is a+b+c
so, maximum perimeter will be 17 + 4b and minimum perimeter will be 17 + 2b
Difference between maximum and minimum is (17 + 4b) - ( 17+2b) = 2b

so our answer will be multiple of 2.
Hence, option B
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When we subtract the two perimeters, the '17' will subtract away. So we'll have our smallest triangle, with sides x and 3x, and our biggest triangle, with sides y and 3y, and the difference in perimeters will be 4y - 4x, which must be even. So only one answer is possible, 12.

To get that answer exactly: if we make x as small as possible, we need the sum of the two shortest sides to exceed the third side. So if our shortest sides are x and 3x, we need their sum, 4x, to be greater than 17, and x must be at least 5.

If we make x as large as possible, we need the sum of the two short sides, x and 17, to exceed the third side, 3x. So x + 17 > 3x must be true, and x can be at most 8 if x is an integer.

So the smallest possible triangle has lengths 5, 15, 17, and the largest has lengths 8, 17, 24, and their perimeters differ by 12.
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