Prove that the greatest angle is 120

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In summary, the sides of the triangle are x^2+3x+3, 2x+3,x^2+2x and the greatest angle is 120°. The greatest angle is determined by the longest side, which can be determined by graphing the three expressions and finding the largest values. It is also important to note that x must be positive for the triangle to be valid.
  • #1
utkarshakash
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Homework Statement


The sides of a triangle are [itex]x^2+3x+3, 2x+3,x^2+2x[/itex]. Prove that the greatest angle of the triangle is 120°.

Homework Equations



The Attempt at a Solution


The greatest angle is the one opposite to the greatest side. But how to decide which one is greatest side? Also the sides are variable.
 
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  • #2
The side lengths must be positive numbers. Is x2+3x+3>x2+2x? Is x2+3x+3>2x+3?

ehild
 
  • #3
Assuming x > 0, it should be obvious which is the longest side. (But you can't say in general which is the shortest.)
Do you know the cosine rule relating lengths of sides of a triangle?
 
  • #4
utkarshakash said:

Homework Statement


The sides of a triangle are [itex]x^2+3x+3, 2x+3,x^2+2x[/itex]. Prove that the greatest angle of the triangle is 120°.

Homework Equations



The Attempt at a Solution


The greatest angle is the one opposite to the greatest side. But how to decide which one is greatest side? Also the sides are variable.
It depends on the value of x. If you graph y1 = x^2 + 3x + 3, y2 = x^2 + 2x, and y3 = 2x + 3, two of the graphs are parabolas and one is a straight line. On the interval [0, 1], the largest values are for y1 = x^2 + 3x + 3 and the smallest are for y2 = x^2 + 2x. For the interval [-1, 0] the order is different.
 
  • #5
Mark44 said:
It depends on the value of x.
No, it turns out not to.
 
  • #6
Mark44 said:
It depends on the value of x.
haruspex said:
No, it turns out not to.
By what I said, I meant that the values of the three expressions depend on x. As you said, one of the sides is the longest, but for other two sides, it depends on which interval you're looking at.
 
  • #7
Turns out x has to be positive, otherwise one of the sides has a negative length.
And if x is positive, there is only one side that can be longest.

I found the results were even more impressive when I worked it through...
 

What does it mean to prove that the greatest angle is 120?

Proving that the greatest angle is 120 means providing evidence or demonstrating that the largest angle in a given set of angles is exactly 120 degrees.

Why is it important to prove that the greatest angle is 120?

Proving that the greatest angle is 120 is important because it helps us understand the properties and relationships of angles, which are fundamental concepts in geometry and other scientific fields.

How can you prove that the greatest angle is 120?

There are several ways to prove that the greatest angle is 120, depending on the given information and context. One way is by using the Triangle Sum Theorem, which states that the sum of the interior angles in a triangle is always 180 degrees. Another way is by using the definition of a straight angle, which is 180 degrees, and the fact that the greatest angle in a triangle is opposite to the longest side.

Can the greatest angle be something other than 120?

Yes, depending on the given information and context, the greatest angle can be something other than 120. In a triangle, the greatest angle can range from 91 degrees (right triangle) to 179 degrees (acute triangle). In other geometric figures, the greatest angle can have different measures.

What are some real-life applications of proving that the greatest angle is 120?

Proving that the greatest angle is 120 has many real-life applications, especially in fields such as architecture, engineering, and navigation. For example, in architecture and engineering, knowing the properties and relationships of angles is essential for designing and constructing structures that are stable and functional. In navigation, understanding angles is crucial for determining direction and position.

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