Question 70372
The most difficult part of this problem is defining the length of one of the equal sides of
the isosceles triangle. Call one of the equal sides L and the base B.
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The problem says that the leg L is 4 times the base B less 8 cm.  In equation form you can
write:
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{{{L = 4*B - 8}}}
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The perimeter of the isosceles triangle is {{{L + L + B}}}. But you now know that {{{L= 4*B - 8}}}.
So, you can substitute {{{4*B - 8}}} for L in the equation for the perimeter to get:
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{{{(4*B-8)+(4*B-8)+B}}} for the perimeter.  This can be simplified by adding all the terms 
containing B and also adding the two terms of -8.  When you do, you end up with:
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{{{9*B - 16}}}
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for the perimeter. But the problem tells you that the perimeter is 29 cm.  Therefore, you
can set the {{{9*B - 16}}} equal to {{{29}}} to get the equation:
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{{{9*B - 16 = 29}}}
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To eliminate the -16 term on the left side, add +16 to both sides.  This results in:
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{{{9*B = 45}}}
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Then divide both sides by 9 to find that {{{B = 5}}}
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You need the answer for the length of one of the legs L.  You already found that this length
is determined by the equation {{{L = 4*B - 8}}}.  All you then need to do is substitute
5 for B and calculate the right side of the equation for L.  You should get the answer that
L = 12.  Work it out to make sure that is the correct answer. And you can check the problem
by adding L plus another L plus the length B to see if you get 29 for the answer.
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Don't forget that you are working in cm so the values for L, B, and the perimeter
all have units of cm.
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Also you need to think through how we got the equation that {{{L = 4*B - 8}}} from the
wording of the problem.
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Hope this helps you to develop skills at analyzing problems such as these word problems.