Monday 25 July 2016

calculus - Value of $int ^infty_0 frac{bsin{ax} - asin{bx}}{x}dx$?



$$\int ^\infty_0 \frac{b\sin{ax} - a\sin{bx}}{x}dx$$




Hello guys! I'm having trouble solving this integral...looks an awful lot like an Frullani Integral, and I've tried to get it to an appropriate form to apply that method, but I've failed so far. Also, Wolfram gives a result lacking any logarithms, which may hint that it cannot be solved in that way. Can anyone help me here? (I have to solve this without using any complex math)


Answer



Hint



Split your integral in txo pieces and make the appropriate changes of variable to make the integrand looking like $$\frac{\sin(y)}{y}~dy$$ and you will then arrive to $$\int \frac{b\sin{ax} - a\sin{bx}}{x}dx=b \text{Si}(a x)-a \text{Si}(b x)$$ from where $$\int ^\infty_0 \frac{b\sin{ax} - a\sin{bx}}{x}dx=\frac{1}{2} \pi \Big(b~~ \text{sgn}(a)-a~~ \text{sgn}(b)\Big)$$


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