SAT Math : How to find the perimeter of an acute / obtuse triangle
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Example Question #2 : Acute / Obtuse Triangles
If a = 7 and b = 4, which of the following could be the perimeter of the triangle?
I. 11
II. 15
III. 25
II and III Only
I and II Only
I, II and III
II Only
I Only
II Only
Consider the perimeter of a triangle:
P = a + b + c
Since we know a and b, we can find c.
In I:
11 = 7 + 4 + c
11 = 11 + c
c = 0
Note that if c = 0, the shape is no longer a trial. Thus, we can eliminate I.
In II:
15 = 7 + 4 + c
15 = 11 + c
c = 4.
This is plausible given that the other sides are 7 and 4.
In III:
25 = 7 + 4 + c
25 = 11 + c
c = 14.
It is not possible for one side of a triangle to be greater than the sum of both of the other sides, so eliminate III.
Thus we are left with only II.
Example Question #141 : Sat Mathematics
Which of the following measurements can NOT represent the sides of a triangle.
Given the Triangle Inequality, the sum of any two sides of a triangle must be greater than the third side.
Given the measurements 
Therefore, these lengths cannot represent a triangle.
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