Quartic functions 2

Level pending

Let f ( x ) = a x 4 + b x 2 + c f(x) = ax^4 + bx^2 + c and g ( x ) = cos ( x ) g(x) = \cos(x) , where f ( 0 ) = g ( 0 ) , f ( π 2 ) = g ( π 2 ) f(0) = g(0), f(\dfrac{\pi}{2}) = g(\dfrac{\pi}{2}) and 0 π 2 f ( x ) d x = 0 π 2 g ( x ) d x \int_{0}^{\dfrac{\pi}{2}} f(x) dx = \int_{0}^{\dfrac{\pi}{2}} g(x) dx .

If the difference D D of the areas of f ( x ) f(x) and g ( x ) g(x) on the interval [ 0 , π 4 ] [0,\dfrac{\pi}{4}] can be expressed as D = α β π + λ β γ β β α β D = \dfrac{\alpha^{\beta}\pi + \lambda - \beta^{\gamma}\sqrt{\beta}}{\beta^{\alpha * \beta}} , where α , β , λ \alpha,\beta,\lambda and γ \gamma are coprime positive integers, find α + β + λ + γ \alpha + \beta + \lambda + \gamma .


The answer is 27.

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1 solution

Rocco Dalto
Apr 17, 2018

f ( 0 ) = g ( 0 ) = 1 c = 1 f(0) = g(0) = 1 \implies c = 1 and f ( π 2 ) = g ( π 2 ) = 0 π 4 a + 4 π 2 b = 16 f(\dfrac{\pi}{2}) = g(\dfrac{\pi}{2}) = 0 \implies \boxed{\pi^4a + 4\pi^2b = -16} and 0 π 2 g ( x ) d x = 0 π 2 cos ( x ) d x = 1 \int_{0}^{\dfrac{\pi}{2}} g(x) dx = \int_{0}^{\dfrac{\pi}{2}} \cos(x) dx = 1 1 = 0 π 2 ( a x 4 + b x 2 + 1 ) d x = \implies 1 = \int_{0}^{\dfrac{\pi}{2}} (ax^4 + bx^2 + 1) dx = a x 5 5 + b x 3 3 + x 0 π 2 \dfrac{ax^5}{5} + \dfrac{bx^3}{3} + x|_{0}^{\dfrac{\pi}{2}} = π 5 5 2 5 a + π 3 3 2 3 b + π 2 = \dfrac{\pi^5}{5 * 2^5}a + \dfrac{\pi^3}{3 * 2^3}b + \dfrac{\pi}{2} \implies

3 π 5 a + 20 π 3 b = 480 240 π \boxed{3\pi^5a + 20\pi^3b = 480 - 240\pi}

π 4 a + 4 π 2 b = 16 \boxed{\pi^4a + 4\pi^2b = -16}

Solving the system above we obtain: a = 80 ( π 3 ) π 5 a = \dfrac{80(\pi - 3)}{\pi^5} and b = 12 ( 5 2 π ) π 3 b = \dfrac{12(5 - 2\pi)}{\pi^3} \implies

0 π 4 f ( x ) d x = 0 π 4 ( 80 ( π 3 ) π 5 x 4 + 12 ( 5 2 π ) π 3 x 2 + 1 ) d x = 16 ( π 3 ) π 5 x 5 + 4 ( 5 2 π ) π 3 x 3 + x 0 π 4 = \int_{0}^{\dfrac{\pi}{4}} f(x) dx = \int_{0}^{\dfrac{\pi}{4}} (\dfrac{80(\pi - 3)}{\pi^5}x^4 + \dfrac{12(5 - 2\pi)}{\pi^3}x^2 + 1) dx = \dfrac{16(\pi - 3)}{\pi^5}x^5 + \dfrac{4(5 - 2\pi)}{\pi^3}x^3 + x|_{0}^{\dfrac{\pi}{4}} =

π 3 4 3 + 5 2 π 4 2 + π 4 = π 3 + 20 8 π + 16 π 64 = 9 π + 17 64 \dfrac{\pi - 3}{4^3} + \dfrac{5 - 2\pi}{4^2} + \dfrac{\pi}{4} = \dfrac{\pi - 3 + 20 - 8\pi + 16\pi}{64} = \dfrac{9\pi + 17}{64}

and,

0 π 4 g ( x ) d x = 0 π 4 cos ( x ) d x = 1 2 \int_{0}^{\dfrac{\pi}{4}} g(x) dx = \int_{0}^{\dfrac{\pi}{4}} \cos(x) dx = \dfrac{1}{\sqrt{2}} \implies

D = 0 π 4 f ( x ) g ( x ) d x = 9 π + 17 64 2 2 = D = \int_{0}^{\dfrac{\pi}{4}} f(x) - g(x) dx = \dfrac{9\pi + 17}{64} - \dfrac{\sqrt{2}}{2} = 9 π + 17 32 2 64 = 3 2 π + 17 2 5 2 2 2 3 = α β π + λ β γ β β α β \dfrac{9\pi + 17 - 32\sqrt{2}}{64} = \dfrac{3^2\pi + 17 - 2^5\sqrt{2}}{2^{2 * 3}} = \dfrac{\alpha^{\beta}\pi + \lambda - \beta^{\gamma}\sqrt{\beta}}{\beta^{\alpha * \beta}} \implies α + β + λ + γ = 27 \alpha + \beta + \lambda + \gamma = \boxed{27} .

Note: I didn't show the graph in the problem, since the graph suggest that the areas are equal on [ 0 , π 4 ] [0,\dfrac{\pi}{4}] which is not the case. As can be seen from the solution the difference is small ( 9 π + 17 32 2 64 0.0003046857 \dfrac{9\pi + 17 - 32\sqrt{2}}{64} \approx 0.0003046857 ).

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