Integrals are just infinite sums

Calculus Level pending

s s e α x t d x \large \int_{-s}^{s} e^{-\alpha x^t}dx Given that the integral above can be expressed as n = 0 C 1 n α n s n t + C 2 n ! ( n t + C 2 ) ( C 2 + C 1 n t ) \sum_{n=0}^{\infty} \frac{C_1^n \alpha^n s^{nt+C_2}}{n!(nt+C_2)}(C_2 + C_1^{nt}) where C 1 C_1 and C 2 C_2 are integer constants between 10 -10 and 10 10 to be found, compute C 2 1 0 C 1 C_2 10^{C_1}


The answer is 0.1.

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1 solution

James Watson
Jan 9, 2021

Note: e x n = 0 x n n ! \displaystyle e^x \equiv \sum_{n=0}^{\infty} \frac{x^n}{n!} I = s s e α x t d x = s s n = 0 ( α x t ) n n ! d x = n = 0 ( 1 ) n α n n ! s s x n t d x = n = 0 ( 1 ) n α n n ! ( x n t + 1 n t + 1 s s ) = n = 0 ( 1 ) n α n n ! ( n t + 1 ) ( s n t + 1 ( 1 ) n t + 1 s n t + 1 ) = n = 0 ( 1 ) n α n s n t + 1 n ! ( n t + 1 ) ( 1 ( 1 ) n t + 1 ) \begin{aligned} I = \int_{-s}^{s}e^{\blue{-\alpha x^t}}dx = \int_{-s}^{s} \sum_{n=0}^{\infty} \frac{(\blue{-\alpha x^t})^n}{n!} dx &= \sum_{n=0}^{\infty} \frac{(-1)^n \alpha^n}{n!}\int_{-s}^{s}x^{nt}dx \\ &= \sum_{n=0}^{\infty} \frac{(-1)^n \alpha^n}{n!}\bigg( \frac{x^{nt+1}}{nt+1}\bigg|_{-s}^{s}\bigg) \\ &= \sum_{n=0}^{\infty} \frac{(-1)^n \alpha^n}{n!(nt+1)}( s^{nt+1} - (-1)^{nt+1}s^{nt+1}) \\ &= \sum_{n=0}^{\infty} \frac{(-1)^n \alpha^n s^{nt+1}}{n!(nt+1)}( 1 - (-1)^{nt+1}) \end{aligned} Since 1 ( 1 ) n t + 1 = 1 1 ( 1 ) n t = ( 1 ) n t -1 \cdot (-1)^{nt+1} = -1 \cdot -1 \cdot (-1)^{nt} = (-1)^{nt} : I = n = 0 ( 1 ) n α n s n t + 1 n ! ( n t + 1 ) ( 1 + ( 1 ) n t ) C 1 = 1 , C 2 = 1 C 2 1 0 C 1 = 1 0 1 = 0.1 I = \sum_{n=0}^{\infty} \frac{(-1)^n \alpha^n s^{nt+1}}{n!(nt+1)}( 1 + (-1)^{nt}) \\ \therefore C_1 = -1, C_2 = 1 \implies C_210^{C_1} = 10^{-1} = \boxed{0.1}

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