Is this a thing?

Geometry Level 3

Is it possible for a triangle with a perimeter of 16 and an area of 11 to be a right triangle?

Yes No

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2 solutions

Sathvik Acharya
Mar 29, 2021

Let a , b a,b be the legs of a right triangle and c c is the hypotenuse. So, we have, a b 2 = 11 a b = 22 ( 1 ) a + b + c = 16 a + b = 16 c ( 2 ) a 2 + b 2 = c 2 ( 3 ) \begin{aligned} \frac{ab}{2}&=11\implies ab=22 &&\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;(1)\\ a+b+c&=16\implies a+b=16-c && \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;(2) \\ a^2+b^2&=c^2 && \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;(3) \end{aligned} Squaring equation ( 2 ) (2) , using equations ( 1 ) (1) and ( 3 ) (3) , ( a + b ) 2 = ( 16 c ) 2 a 2 + b 2 + 2 a b = 256 32 c + c 2 c 2 + 44 = 256 32 c + c 2 32 c = 212 c = 53 8 \begin{aligned} (a+b)^2&=(16-c)^2 \\ a^2+b^2+2ab&=256-32c+c^2 \\ c^2+44&=256-32c+c^2 \\ 32c&=212 \\ \therefore \; c&=\frac{53}{8} \end{aligned} So equation ( 2 ) (2) transforms to, a + b = 75 8 a + 22 a = 75 8 8 a 2 75 a + 176 = 0 \begin{aligned} a+b&=\frac{75}{8} \\ a+\frac{22}{a}&=\frac{75}{8} \\ 8a^2-75a+176&=0 \\ \end{aligned} In the above quadratic, the discriminant , Δ = ( 75 ) 2 4 8 176 = 7 < 0 \Delta=(-75)^2-4\cdot 8\cdot 176=-7<0 . Hence, the quadratic does not have real solutions and both a a , b b are complex numbers.

Therefore, there does not exist a right triangle such that its area is 11 11 and perimeter is 16 16 .

David Vreken
Mar 29, 2021

Assume that it is possible, and let a a and b b be the legs of the right triangle.

By the Pythagorean Theorem, the hypotenuse is a 2 + b 2 \sqrt{a^2 + b^2} , so the perimeter would be P = a + b + a 2 + b 2 = 16 P = a + b + \sqrt{a^2 + b^2} = 16 .

Also, the area would be A = 1 2 a b = 11 A = \frac{1}{2}ab = 11 , which rearranges to b = 22 a b = \frac{22}{a} .

Substituting b = 22 a b = \frac{22}{a} into a + b + a 2 + b 2 = 16 a + b + \sqrt{a^2 + b^2} = 16 gives a + 22 a + a 2 + ( 22 a ) 2 = 16 a + \frac{22}{a} + \sqrt{a^2 + (\frac{22}{a})^2} = 16 , which rearranges to 8 a 2 75 a + 176 = 0 8a^2 - 75a + 176 = 0 (for a 0 a \neq 0 ).

However, the discriminant of 8 a 2 75 a + 176 = 0 8a^2 - 75a + 176 = 0 is 7 5 2 4 8 176 = 7 75^2 - 4 \cdot 8 \cdot 176 = -7 , so there is no real solution for a a .

The original assumption leads to a contradiction, so we can reject it and conclude that such a triangle is not possible .

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