Quartic functions and Trigonometric functions 2

Calculus Level 4

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

If the area A A of the region bounded between by the curves f ( x ) f(x) and g ( x ) g(x) on [ π 4 , 0 ] \left[-\frac{\pi}{4},0\right] can be expressed as A = α β β λ + γ λ π β α β A = \dfrac{\alpha^{\beta}}{\beta^{\lambda}} + \dfrac{\gamma \lambda\pi}{\beta^{\alpha}} - \sqrt{\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 17.

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1 solution

Rocco Dalto
Apr 23, 2018

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

2 = 0 π 2 ( a x 4 + b x 2 + 1 ) d x = a 5 x 5 + b 3 x 3 + x 0 π 2 = π 5 5 2 5 a + π 3 3 2 3 + π 2 2 = \int_{0}^{\dfrac{\pi}{2}} (ax^4 + bx^2 + 1) dx = \dfrac{a}{5}x^5 + \dfrac{b}{3}x^3 + x|_{0}^{\dfrac{\pi}{2}} = \dfrac{\pi^5}{5 * 2^5}a + \dfrac{\pi^3}{3 * 2^3} + \dfrac{\pi}{2} \implies

3 π 5 a + 20 π 3 b = 960 240 π \boxed{3\pi^5a + 20\pi^3b = 960 - 240\pi}

π 4 a + 4 π 2 b = 0 \boxed{\pi^4a + 4\pi^2b = 0}

Solving the above system we obtain:

a = 120 ( π 4 π 5 ) a = 120(\dfrac{\pi - 4}{\pi^5}) and b = 30 ( 4 π π 3 ) b = 30(\dfrac{4 - \pi}{\pi^3}) \implies

π 4 0 f ( x ) d x = π 4 0 ( 120 ( π 4 π 5 ) x 4 + 30 ( 4 π π 3 ) x 2 + 1 ) d x = 24 ( π 4 π 5 ) x 5 + 10 ( 4 π π 3 ) x 3 + x π 4 0 = \int_{\dfrac{-\pi}{4}}^{0} f(x) dx = \int_{\dfrac{-\pi}{4}}^{0} (120(\dfrac{\pi - 4}{\pi^5})x^4 + 30(\dfrac{4 - \pi}{\pi^3})x^2 + 1) dx = 24(\dfrac{\pi - 4}{\pi^5})x^5 + 10(\dfrac{4 - \pi}{\pi^3})x^3 + x|_{\dfrac{-\pi}{4}}^{0} =

6 4 4 ( π 4 ) + 10 4 3 ( 4 π ) + π 4 = 6 ( π 4 ) + 40 ( 4 π ) + 64 π 256 = 30 π + 136 256 = 15 π + 68 128 \dfrac{6}{4^4}(\pi - 4) + \dfrac{10}{4^3}(4 - \pi) + \dfrac{\pi}{4} = \dfrac{6(\pi - 4) + 40(4 - \pi) + 64\pi}{256} = \dfrac{30\pi + 136}{256} = \dfrac{15\pi + 68}{128}

and,

π 4 0 g ( x ) d x = π 4 0 sin ( x ) + cos ( x ) d x = sin ( x ) cos ( x ) π 4 0 = 2 1 \int_{\dfrac{-\pi}{4}}^{0} g(x) dx = \int_{\dfrac{-\pi}{4}}^{0} \sin(x) + \cos(x) dx = \sin(x) - \cos(x)|_{\dfrac{-\pi}{4}}^{0} = \sqrt{2} - 1

π 4 0 f ( x ) g ( x ) d x = 15 π + 68 128 ( 2 1 ) = 196 128 + 15 π 128 2 = 49 32 + 15 π 128 2 = \implies \int_{\dfrac{-\pi}{4}}^{0} f(x) - g(x) dx = \dfrac{15\pi + 68}{128} - (\sqrt{2} - 1) = \dfrac{196}{128} + \dfrac{15\pi}{128} - \sqrt{2} = \dfrac{49}{32} + \dfrac{15\pi}{128} - \sqrt{2} = 7 2 2 5 + 3 5 π 2 7 2 = α β β λ + γ λ π β α β \dfrac{7^2}{2^5} + \dfrac{3 * 5\pi}{2^7} - \sqrt{2} = \dfrac{\alpha^{\beta}}{\beta^{\lambda}} + \dfrac{\gamma * \lambda\pi}{ \beta^{\alpha}} - \sqrt{\beta} \implies α + β + λ + γ = 17 \alpha + \beta + \lambda + \gamma = \boxed{17} .

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