Triangular numbers to altitudes

Geometry Level 4

In triangle A B C ABC with angles A = a A=a , B = c B=c , and C = b C=b , ( 3 a 2 b + 4 c 105 x ) (3a-2b+4c-105x) , ( 5 b 2 c + ( 2 a / 3 ) 30 x ) (5b-2c+(2a/3)-30x) , and ( 5 a 3 b + ( 6 c 5 (5a-3b+(\frac{6c}{5} )+( x 2 \frac{x}{2} )) are consecutive terms in the triangular number sequence, where x x is an integer and the sum of the terms is 361 361 . If the ratio of the altitude to A C : AC : altitude to B C : BC : altitude to A B AB is in the form p : 1 : q p : 1 : q , determine the value of p q pq . If this product can be expressed as ( f m ) ( h h m ) ( h ) \frac{(f^m)(h-h^m)}{(h)} , find the value of f m h fmh .


The answer is 3.

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1 solution

Yashas Ravi
Jun 19, 2018

Use the formula that the nth triangular number is equal to n ( n + 1 ) 2 \frac{n(n+1)}{2} to determine that the consecutive terms are the 14th, 15th, and 16th, or 105 105 , 120 120 , and 136 136 . Setting the 3 given expressions equal to their respective terms and using the fact that the angles of a triangle ( a + b + c a+b+c ) equals 180 180 , it can be derived that a = 45 a=45 , b = 60 b=60 , and c = 75 c=75 . Using the Law of Sines, we can find the ratio of sides. Finally, using the fact that the area of a triangle equals the base times the height divided by 2 2 , the ratio of altitudes is p = ( 3 1 ) : 1 : q = ( 6 ) 3 p=(\sqrt{3}-1) : 1 : q=\frac{(\sqrt{6})}{3} ). As a result, p q = 2 ( 3 3 ) 3 pq=\frac{\sqrt{2}(3-\sqrt{3})}{3} ), and since the square root of integer k k is k k to the ( 1 ) 2 \frac{(1)}{2} power, 0.5 3 2 = 3 0.5*3*2=3 , which is our final answer.

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