TRANSFORM OF AN INTEGRAL

The transform of an integral can readily be found from that derived above for a derivative. Let the integral be

у — jx(t) dt

and let it be required to find y(s). By differentiating with respect to t, we get

TRANSFORM OF AN INTEGRAL TRANSFORM OF AN INTEGRAL Подпись: (2.3Д6)

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EXTREME VALUE THEOREMS

Equation (2.3,14) may be rewritten as

Л0О

—x(0) + sx(s) = I e~stx(t) dt rT

= lim e~stx(t) dt

T-* oo Jo

TRANSFORM OF AN INTEGRAL

We now take the limit s —► 0 while T is held constant, i. e.

CT

Подпись: Hence Подпись: lim sx(s) = lim x(T) s-^0 T~* oo Подпись: (2.3,17)

= lim x(t) dt = lim x(T) — a;(0)] T-* oo Jo T-* 00

This result, known as the final value theorem, provides a ready means for determining the asymptotic value of x(t) for large times from the value of its Laplace transform.

In a similar way, by taking the limit s —*■ oo at constant T, the integral vanishes for all finite x(t) and we get the initial value theorem.

Подпись: (2.3,18)lim sx(s) = x(0)

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