Suppose a triangle has two sides of length 3and 4 and that the angle between these two sides is 60. What is the length of the third side of the triangle?

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  A.  √13

Step-by-step explanation:

You can make an educated guess and come to the right conclusion.

The triangle is nearly an equilateral triangle. A triangle with two sides 3 and an angle of 60° would have a third side of 3. A triangle with two sides of 4 and an angle of 60° would have a third side of 4.

So, the third side must be between 3 and 4. Here is an evaluation of the answer choices:

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A -- between 3 and 4, the correct choice

B -- 3, too short

C -- 1.73, too short

D -- more than 4, too long

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The question can be answered using your triangle solver app on your calculator, or using the Law of Cosines.

  c = √(a^2 +b^2 -2ab·cos(C))

  c = √(3^2 +4^2 -2·3·4·(1/2)) = √(9 +16 -12)

  c = √13 . . . . . length of the side opposite the 60° angle

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