Cubic and Trig functions 2

Calculus Level 4

Let f ( x ) = x 3 + b x 2 + c x + d f(x) = x^3 + bx^2 + cx + d and g ( x ) = cos ( x ) sin ( x ) g(x) = \cos(x) - \sin(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 g ( x ) g(x) and f ( x ) f(x) on [ π 4 , 0 ] \left[-\frac{\pi}{4},0\right] can be expressed as A = α β ( π β ) β α β λ π + ( β β ) γ ( β β ) γ A = \dfrac{\alpha^{\beta}(\pi^{\beta})^{\beta} - \alpha\beta^{\lambda}\pi + (\beta^{\beta})^{\gamma}}{(\beta^{\beta})^{\gamma}} , 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
May 24, 2018

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

0 = 0 π 2 ( x 3 + b x 2 + c x + 1 ) d x = 1 4 x 4 + b 3 x 3 + c 2 x 2 + x 0 π 2 = π 4 2 6 + π 3 3 2 3 b + π 2 2 3 c + π 2 0 = \int_{0}^{\dfrac{\pi}{2}} (x^3 + bx^2 + cx + 1) dx = \dfrac{1}{4}x^4 + \dfrac{b}{3}x^3 + \dfrac{c}{2}x^2 + x|_{0}^{\dfrac{\pi}{2}} = \dfrac{\pi^4}{2^6} + \dfrac{\pi^3}{3 * 2^3}b + \dfrac{\pi^2}{2^3}c + \dfrac{\pi}{2} \implies

8 π 3 b + 24 π 2 c = 96 π 3 π 4 \boxed{8\pi^3b + 24\pi^2c = - 96\pi - 3\pi^4}

2 π 2 b + 4 π c = 16 π 3 \boxed{2\pi^2 b + 4\pi c = -16 - \pi^3}

Solving the above system we obtain:

c = π 3 32 8 π c = \dfrac{\pi^3 - 32}{8\pi} and b = 3 π 4 b = -\dfrac{3\pi}{4} \implies

π 4 0 f ( x ) d x = x 4 4 π 4 x 3 + π 3 32 16 π x 2 + x π 4 0 = 384 π 9 π 4 1024 \int_{\dfrac{-\pi}{4}}^{0} f(x) dx = \dfrac{x^4}{4} - \dfrac{\pi}{4} x^3 + \dfrac{\pi^3 - 32}{16\pi} x^2 +x|_{\dfrac{-\pi}{4}}^{0} = \dfrac{384\pi - 9\pi^4}{1024}

and

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

π 4 0 g ( x ) f ( x ) d x = 9 π 4 384 π + 1024 1024 = \implies \int_{\dfrac{-\pi}{4}}^{0} g(x) - f(x) \:\ dx = \dfrac{9\pi^4 - 384\pi + 1024}{1024} = 3 2 ( π 2 ) 2 3 2 7 π + ( 2 2 ) 5 ( 2 2 ) 5 = α β ( π β ) β α β λ π + ( β β ) γ ( β β ) γ \dfrac{3^2(\pi^{2})^2 - 3 * 2^7\pi + (2^{2})^{5}}{(2^{2})^5} = \dfrac{\alpha^{\beta}(\pi^{\beta})^{\beta} - \alpha\beta^{\lambda}\pi + (\beta^{\beta})^{\gamma}}{(\beta^{\beta})^{\gamma}} \implies α + β + λ + γ = 17 \alpha + \beta + \lambda + \gamma = \boxed{17} .

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