# _-substitution: definite integral of exponential function | AP Calculus AB | Khan Academy | Summary and Q&A

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April 30, 2014
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_-substitution: definite integral of exponential function | AP Calculus AB | Khan Academy

## TL;DR

The video explains how to calculate the definite integral of x^2 * 2^(x^3) from 0 to 1.

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### Q: How is the base 2 re-expressed as e in the problem?

The base 2 is re-expressed as e by finding the natural logarithm of 2, which is approximately 0.693. So, 2 can be represented as e^(0.693).

### Q: Why is u-substitution used in this problem?

U-substitution is used to simplify the expression and make it easier to find the anti-derivative. By defining u = x^3 * ln(2), we can rewrite the expression in terms of u and its derivative du.

### Q: What is the anti-derivative of e^u?

The anti-derivative of e^u is simply e^u, as mentioned in the video. So, the integral of e^u du is equal to e^u + C, where C is the constant of integration.

### Q: How is the definite integral calculated from 0 to 1?

To calculate the definite integral, you need to evaluate the anti-derivative at the upper limit (1) and subtract the evaluation at the lower limit (0). This eliminates the constant of integration and gives you the final result.

## Summary & Key Takeaways

• The video begins by discussing the need to re-express the base 2 as e in order to simplify the problem.

• Using u-substitution, the video transforms the expression into the anti-derivative of e^u, where u = x^3 * ln(2).

• After finding the anti-derivative, the video evaluates it at 1 and subtracts the evaluation at 0 to obtain the final result of 1/natural log(8).