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I need help with altitudes

 

Triangle ABC has altitudes AD, BE, and CF.  If AD = 12, BE = , and CF is a positive integer, then find the largest possible value of CF.

 May 30, 2023
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We can use the property that the product of the lengths of the altitudes of a triangle is equal to the product of its semiperimeter and its inradius. In other words:

AD * BE * CF = s * r

where s is the semiperimeter of the triangle and r is its inradius. Since CF is the largest of the three altitudes, we want to find the largest possible value of CF, which means we want to maximize the right-hand side of this equation.

Let's label the sides of triangle ABC as a, b, and c, with opposite vertices A, B, and C, respectively. Then, the semiperimeter s is:

s = (a + b + c) / 2

The inradius can be found using the formula:

r = A / s

where A is the area of the triangle. We can find the area of the triangle using any of the altitudes, so let's use AD:

A = (1/2) * AD * BC

Substituting the given values, we get:

A = (1/2) * 12 * BC = 6BC

Now we can substitute the expressions for s and r into the equation above:

AD * BE * CF = s * r
12 * BE * CF = [(a + b + c) / 2] * (6BC / (a + b + c))

Simplifying, we get:

24BE * CF= 3BC * 6BE

Dividing both sides by 6BE, we get:

4CF = BC

To maximize CF, we want BC to be as large as possible. However, we also know that BC must be less than the sum of the other two sides, since it is opposite the largest angle of the triangle. Therefore, we want BC to be as close as possible to the sum of the other two sides.

Let's assume that the sum of the other two sides is equal to 2x, where x is a positive integer. Then, BC must be less than 2x. We want BC to be as close as possible to 2x, so we choose BC = 2x - 1. This ensures that BC is as large as possible while still being less than 2x.

Now we can substitute this value of BC into the equation we derived earlier:

4CF = BC = 2x - 1

Since x is a positive integer, the largest possible value of BC is when x = 5, which gives BC = 9. Therefore, the largest possible value of CF is:

4CF = 2x - 1 = 9 * 4 = 36

So the largest possible value of CF is 36. 

 May 31, 2023

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