Big Ideas Math Geometry, 2014
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Big Ideas Math Geometry, 2014 View details
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Exercise 5 Page 523

Compare the square of the largest side length to the sum of the squares of the other two side lengths.

Do the Segment Lengths Form a Triangle? Yes.
Is the Triangle Acute, Right, or Obtuse? Acute triangle.

Practice makes perfect

We have been given the following segment lengths. We will investigate whether these segment lengths form a triangle. Then, we will determine whether the triangle is acute, right, or obtuse. 4, sqrt()67, and 9 Let's start!

Step I

In this step, we will use the Triangle Inequality Theorem to verify that the segment lengths form a triangle. However, we first need to have the approximate value of the given radical number to compare the segment lengths. To do so, let's estimate it by looking at nearby perfect squares.


64 < 67 < 81 ⇕ 8^2< ( sqrt(67))^2 < 9^2

Notice that, in the perfect square form, 67 is closer to 64. Therefore, the approximate value of sqrt(67) can be found as follows.

sqrt(67) &≈ 8.2

Great! Now, we can use the theorem to verify that the segment lengths form a triangle.

Inequality Check
4+ sqrt(67) ? > 9
≈ 12.2 > 9 ✓
4+9 ? > sqrt(67) 13 > ≈ 8.2 ✓
sqrt(67) + 9 ? > 4 ≈ 17.2 > 4 ✓

We can see that the segments with lengths of 4, sqrt(67), and 9 satisfy the Triangle Inequality Theorem. Therefore, they form a triangle.

Step II

We want to determine whether the triangle formed by the given side lengths is acute, right, or obtuse. To do so, we will compare the square of the largest side length to the sum of the squares of the other two side lengths. Let a, b, and c be the lengths of the sides, with c being the longest.

Condition Type of Triangle
a^2+b^2 < c^2 Obtuse triangle
a^2+b^2 = c^2 Right triangle
a^2+b^2 > c^2 Acute triangle
Let's now consider the given side lengths 4, sqrt(67), and 9. Since 9 is the greatest of the numbers, we will let c be 9. We will also arbitrarily let a be 4 and b be sqrt(67). 4^2+( sqrt(67))^2 ? 9^2 Let's simplify the above statement to determine whether the left-hand side is less than, equal to, or greater than the right-hand side.
4^2+( sqrt(67))^2 ? 9^2
16+67 ? 81
83 > 81
Referring back to our table, we can conclude that the side lengths 4, sqrt(67), and 9 form an acute triangle.