Equal Areas 2

Calculus Level 4

Let x < 1 |x| < 1 and e e be Euler's number .

Let f ( x ) = lim n j = 1 n x j j = 1 n ( j n ) n x n j f(x) = \lim_{n \rightarrow \infty} \dfrac{\sum_{j = 1}^{n} x^{j}}{\sum_{j = 1}^{n} (\dfrac{j}{n})^n x^{n - j}} and g ( x ) = a x 2 + b x + c g(x) = ax^2 + bx + c , where lim x 0 f ( x ) = lim x 0 g ( x ) , f ( e 2 ) = g ( e 2 ) \lim_{x \rightarrow 0} f(x) =\lim_{x \rightarrow 0} g(x), f(e - 2) = g(e - 2) and 0 e 2 f ( x ) d x = 0 e 2 g ( x ) d x \int_{0}^{e - 2} f(x) dx = \int_{0}^{e - 2} g(x) dx .

Find the area of the region bounded by f ( x ) f(x) and g ( x ) g(x) on [ 2 e , 0 ] [2 - e,0] to eight decimal places.

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The answer is 0.69334499.

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1 solution

Rocco Dalto
Jun 1, 2018

lim n j = 1 n ( j n ) n x n j = j = 0 n 1 ( 1 j n ) n x j = n = 0 ( x e ) n = e e x \lim_{n \rightarrow \infty} \sum_{j = 1}^{n} (\dfrac{j}{n})^n x^{n - j} = \sum_{j = 0}^{n - 1} (1 - \dfrac{j}{n})^{n} x^{j} = \sum_{n = 0}^{\infty} (\dfrac{x}{e})^n = \dfrac{e}{e - x} on x < e |x| < e

f ( x ) = lim n j = 1 n x j j = 1 n ( j n ) n x n j = ( x 1 x ) ( e x e ) = x ( e x ) e ( 1 x ) \implies f(x) = \lim_{n \rightarrow \infty} \dfrac{\sum_{j = 1}^{n} x^{j}}{\sum_{j = 1}^{n} (\dfrac{j}{n})^n x^{n - j}} = (\dfrac{x}{1 - x})(\dfrac{e - x}{e}) = \dfrac{x(e - x)}{e(1 - x)} on x < 1 |x| < 1 .

lim x 0 f ( x ) = lim x 0 g ( x ) = 0 c = 0 \lim_{x \rightarrow 0} f(x) =\lim_{x \rightarrow 0} g(x) = 0 \implies c = 0 and f ( e 2 ) = g ( e 2 ) = 2 ( e 2 ) e ( 3 e ) f(e - 2) = g(e - 2) = \dfrac{2(e - 2)}{e(3 - e)} \implies ( e 2 ) a + b = 2 e ( 3 e ) \boxed{(e - 2)a + b = \dfrac{2}{e(3 - e)}}

0 e 2 g ( x ) = 0 e 2 f ( x ) = 1 e 0 e 2 ( x + e 1 1 x ( e 1 ) ) d x = 1 2 e ( x 2 2 ( e 1 ) ( ln ( 1 x ) + x ) ) 0 e 2 = \int_{0}^{e - 2} g(x) = \int_{0}^{e - 2} f(x) = \dfrac{1}{e}\int_{0}^{e - 2} (x + \dfrac{e - 1}{1 - x} - (e - 1)) \:\ dx = \dfrac{1}{2e}(x^2 - 2(e - 1)(\ln(1 - x) + x))|_{0}^{e - 2} =

1 2 e ( 2 ( e 1 ) ln ( 1 3 e ) e ( e 2 ) ) \dfrac{1}{2e}(2(e - 1)\ln(\dfrac{1}{3 - e}) -e(e - 2))

1 2 e ( 2 ( e 1 ) ln ( 1 3 e ) e ( e 2 ) ) = 0 e 2 g ( x ) d x = a ( e 2 ) 3 3 + b ( e 2 ) 2 2 \implies \dfrac{1}{2e}(2(e - 1)\ln(\dfrac{1}{3 - e}) -e(e - 2)) = \int_{0}^{e - 2} g(x) \:\ dx = \dfrac{a(e - 2)^3}{3} + \dfrac{b(e - 2)^2}{2} \implies

2 ( e 2 ) a + 3 b = 3 e ( e 2 ) 2 ( 2 ( e 1 ) ln ( 1 3 e ) e ( e 2 ) ) \boxed{2(e - 2)a + 3b = \dfrac{3}{e(e - 2)^2}(2(e - 1)\ln(\dfrac{1}{3 - e}) - e(e - 2))}

and

( e 2 ) a + b = 2 e ( 3 e ) \boxed{(e - 2)a + b = \dfrac{2}{e(3 - e)}}

Solving the system we obtain:

a = 3 ( e 1 ) e ( e 3 ) ( e 2 ) 3 ( ( e 2 ) ( e 4 ) 2 ( 3 e ) ln ( 3 e ) ) a = \dfrac{3(e - 1)}{e(e - 3)(e - 2)^3}((e - 2)(e - 4) - 2(3 - e)\ln(3 - e))

and

b = 1 e ( e 3 ) ( e 2 ) 2 ( ( e 2 ) ( 3 e 2 13 e + 8 ) 6 ( e 1 ) ( 3 e ) ln ( 3 e ) ) b = -\dfrac{1}{e(e - 3)(e - 2)^2}((e - 2)(3e^2 - 13e + 8) - 6(e - 1)(3 - e)\ln(3 - e))

2 e 0 f ( x ) d x = 1 2 e ( x 2 2 ( e 1 ) ( ln ( 1 x ) + x ) ) 2 e 0 = \int_{2 - e}^{0} f(x) \:\ dx = \dfrac{1}{2e}(x^2 - 2(e - 1)(\ln(1 - x) + x))|_{2 - e}^{0} = 1 2 e ( ( 3 e 4 ) ( e 2 ) 2 ( e 1 ) ln ( e 1 ) ) . 206757934 -\dfrac{1}{2e}((3e - 4)(e - 2) - 2(e - 1)\ln(e - 1)) \approx \boxed{-.206757934}

2 e 0 g ( x ) d x = 1 6 ( 2 a x 3 + 3 b x 2 ) 2 e 0 = 1 6 ( 2 a ( e 2 ) 3 3 b ( e 2 ) 2 ) = \int_{2 - e}^{0} g(x) \:\ dx = \dfrac{1}{6}(2ax^3 + 3bx^2)|_{2 - e}^{0} = \dfrac{1}{6}(2a(e - 2)^3 - 3b(e - 2)^2) =

1 2 e ( e 3 ) ( ( e 2 ) ( 5 e 2 23 e + 16 ) 10 ( e 1 ) ( 3 e ) ln ( 3 e ) ) . 48658699 \dfrac{1}{2e(e - 3)}((e - 2)(5e^2 - 23e + 16) - 10(e - 1)(3 - e)\ln(3 - e)) \approx \boxed{.48658699}

2 e 0 g ( x ) f ( x ) d x 0.69334499 \implies \int_{2 - e}^{0} g(x) - f(x) \:\ dx \approx \boxed{0.69334499}

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