The Largest Angle

Geometry Level 2

One angle of a triangle is 2 4 24^{\circ} less than twice the sum of the second and third angles. Find the measure (in degrees) of the largest angle in the triangle.

108 106 112 110

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2 solutions

Arron Kau Staff
May 13, 2014

Denote the three angles in the triangle by A A , B B , and C C . Then the three angles must add up to 18 0 180^{\circ} : A + B + C = 180 A+ B+ C = 180 . Furthermore, the statement given in the problem can be translated to the equation A = 2 ( B + C ) 24 A = 2(B + C) - 24 . We can solve the equation A + B + C = 180 A+ B+ C = 180 for C C , getting C = 180 A B C = 180 - A - B , and substitute this in the other equation to get A = 2 ( B + 180 A B ) 24 A = 2(B + 180 - A - B) - 24 . This equation simplifies to A = 360 2 A 24 A = 360 - 2A - 24 , and then to 3 A = 336 3A = 336 , so we get A = 112 A = 112 . Since this angle is greater than 9 0 90^{\circ} , it must be the largest angle. Therefore, the measure of the largest angle is 112 degrees.

Rick B
Oct 16, 2014

{ α + β + γ = 180 α = 2 ( β + γ ) 24 \begin{cases} \alpha + \beta + \gamma = 180 \\ \alpha = 2(\beta + \gamma)-24 \end{cases}

Substitution of α \alpha in equation 1:

2 ( β + γ ) 24 + β + γ = 180 2(\beta + \gamma)-24 + \beta + \gamma = 180

3 ( β + γ ) = 204 β + γ = 204 3 = 68 \implies 3(\beta + \gamma) = 204 \implies \beta + \gamma = \frac{204}{3} = 68

Knowing that β + γ = 68 \beta + \gamma = 68 , it's obvious that α \alpha is the largest angle, and its value is 180 68 = 11 2 180-68 = \boxed{112^\circ}

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