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The factor is essential because the Fundamental Theorem of Calculus combined with the Chain Rule requires multiplying the integrand evaluated at the upper limit by the derivative of that limit. Without this factor, the rate of change of the accumulated area with respect to the outer variable $t$ would be incorrect.

Conditions: The upper limit is a function of $t$, not just $t$ itself.; The integrand is continuous.

The factor is essential because the Fundamental Theorem of Calculus combined with the Chain Rule requires multiplying the integrand evaluated at the upper limit by the derivative of that limit. Without this factor, the rate of change of the accumulated area with respect to the outer variable $t$ would be incorrect.

Conditions: The upper limit is a function of $t$, not just $t$ itself.; The integrand is continuous.