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Math Help - Number of Isosceles triangles

  1. #1
    Senior Member pankaj's Avatar
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    Number of Isosceles triangles

    If an isosceles triangle has positive integers as its lengths then find the number of such triangles none of whose lengths exceed 1994
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    Re: Number of Isosceles triangles

    When you say "lengths" you mean the total length of all three sides, right? An isosceles triangle has two sides of equal length and another side. Calling the two equal lengths "x" and the third side "y", the total length is 2x+ y so you are looking for all possible integers x and y such that 2x+ y= 1994. So y= 1994- 2x= 2(997- x) which means that y must be an even number.

    In order to have 3 sides, x and y must be positive (not 0) so x can be any integer from 1 to 996 which then gives a unique value for y.
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    Re: Number of Isosceles triangles

    There are two types of triangles to consider
    1 equal legs 1994
    2 base 1994
    Count 1 1993
    Count 2 998
    Total 2991
    Last edited by bjhopper; July 18th 2013 at 01:09 PM. Reason: missing word
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    MHF Contributor ebaines's Avatar
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    Re: Number of Isosceles triangles

    There are a lot more possibilities than these. Assuming that the total length of all sides may not exceed 1994, this does not mean that the total length has to equal 1994. For example a triangle with legs of 1, 1, and 2 satisfies the requirement.

    Let y = length of the base and x = the lengths of the two equal sides. The requirement is that 2x + y <= 1994. Let's look at what happens for different values of y:

    If y = 1, the x can be any value from 1 to (1994-1)/2 rounded down to an integer, or 996
    if y= 2 then x can be any value from 1 to (1994-2)/2 = 996
    if y = 3 then x can be any integer from 1 to 995
    if y = 4 then x can be any integer from 1 to 995
    etc.
    ...
    if y = 1989 the x can be any integer from 1 to 2
    if y = 1990 then x can be any value from 1 to 2
    if y = 1991 then x = 1
    if y = 1992 then x = 1.

    So the total of all possibilities is  2 \times \sum_{i=1} ^{996} i = 2 \times \frac {996 \times 997} 2 = 993,012.

    However, given the strange wording of the question perhaps OP means that no single leg may exceed length of 1994. If that's what he meant then there are 1994 choices for length of leg x and 1994 choices for leg y, so the total number of possibilities is 1994 x 1994 = 3,976,036.
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    Re: Number of Isosceles triangles

    Quote Originally Posted by ebaines View Post
    There are a lot more possibilities than these. Assuming that the total length of all sides may not exceed 1994, this does not mean that the total length has to equal 1994. For example a triangle with legs of 1, 1, and 2 satisfies the requirement.

    Let y = length of the base and x = the lengths of the two equal sides. The requirement is that 2x + y <= 1994. Let's look at what happens for different values of y:

    If y = 1, the x can be any value from 1 to (1994-1)/2 rounded down to an integer, or 996
    if y= 2 then x can be any value from 1 to (1994-2)/2 = 996
    if y = 3 then x can be any integer from 1 to 995
    if y = 4 then x can be any integer from 1 to 995
    etc.
    ...
    if y = 1989 the x can be any integer from 1 to 2
    if y = 1990 then x can be any value from 1 to 2
    if y = 1991 then x = 1
    if y = 1992 then x = 1.

    So the total of all possibilities is  2 \times \sum_{i=1} ^{996} i = 2 \times \frac {996 \times 997} 2 = 993,012.

    However, given the strange wording of the question perhaps OP means that no single leg may exceed length of 1994. If that's what he meant then there are 1994 choices for length of leg x and 1994 choices for leg y, so the total number of possibilities is 1994 x 1994 = 3,976,036.
    I followed your last sentence and erred my counts should have been 1993 and 1993 but aren't they additive.You are notallowed to make an equilateral triangle for example
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