Big Ideas Math Geometry, 2014
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Big Ideas Math Geometry, 2014 View details
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Exercise 6 Page 486

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. 12, 15, and 10sqrt(3) 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 an approximate value of 10sqrt(3) to be able to compare segment lengths. To do so, let's estimate them by looking at nearby perfect squares.

289< 300 <324 ⇕ 17^2< (10 sqrt(3))^2 < 18^2 We know that 10 sqrt(3) is somewhere between 17 and 18. Let's say that 10 sqrt(3) is approximately equal to 17.2 because, in the perfect square form, 300 is closer to 289 rather than 324. 10sqrt(3) &≈ 17.2 Great! Now, we can use the theorem to verify that the segment lengths form a triangle.

Inequality Check
12+15 ? > 10sqrt(3)
27 >≈ 17.2 ✓
12+10sqrt(3) ? > 15 ≈ 29.2 > 24 ✓
10sqrt(3)+15 ? > 12 ≈ 32.2>32 ✓

We can see that segments with lengths of 12, 15, and 10sqrt(3) 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 12, 15, and 10sqrt(3). Since 10sqrt(3) is the greatest of the numbers, we will let c be 10sqrt(3). We will also arbitrarily let a be 12 and b be 15. 12^2+15^2 ? ( 10sqrt(3) ) ^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.
12^2+15^2 ? ( 10sqrt(3) ) ^2
144 + 225 ? 300
369 > 300
Referring back to our table, we can conclude that the side lengths 12, 15, and 10sqrt(3) form an acute triangle.