Rational function with Trig coefficients

Algebra Level pending

F ( x ) = x 2 + x 7 sin θ + cos θ x 2 + x + 4 cos θ + sin θ F(x) = \frac {x^2+|x-7|\sin \theta + \cos \theta}{x^2+|x+4|\cos \theta + \sin \theta}

If 0 < θ < 9 0 0^\circ< \theta <90^\circ and F ( x ) F(x) intersects its horizontal asymptote only one time, the intersection is at ( a , b ) (a,b) where a a and b b are reals. Determine the value of 10 a + 5 b 10a+5b .

Note : Please refrain from using a graphing calculator.


The answer is 20.

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1 solution

Yashas Ravi
Apr 10, 2020

The horizontal asymptote would be y = 1 y=1 since the graph would approach the ratio of the leading coefficients, which is 1 1 , as x approaches infinity. If we let F ( x ) = 1 F(x)=1 , then x 2 + sin ( θ ) x 7 + cos ( θ ) = x 2 + cos ( θ ) x + 4 + sin ( θ ) x^2+\sin(θ)|x-7|+\cos(θ)=x^2+\cos(θ)|x+4|+\sin(θ) . By canceling out the x x and factoring, tan ( θ ) = \tan(θ)= 1 x + 4 1 x 7 \frac{1-|x+4|}{1-|x-7|} . Since there is only 1 1 possible asymptote, that means there is 1 1 possible value for x x , so x + 4 = x 7 1 |x+4|=|x-7|≠1 and θ = 45 ° θ=45° . This yields only x + 4 = 7 x x+4=7-x as x + 4 = x 7 x+4=x-7 has no solution, so 2 x = 3 2x=3 and x = 1.5 x=1.5 . As a result, the intersection is ( 1.5 , 1 ) (1.5,1) so 10 ( 1.5 ) + 5 ( 1 ) = 20 10(1.5)+5(1)=20 which is the final answer.

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