Section 14: Numerical Integration 201

Because the accuracy of any integral is limited by the accuracy of the function (as indicated in the display format), the calculator cannot compute the value of an integral exactly, but rather only approximates it. The HP-15C places the uncertainty* of an integral's approximation in the Y- register at the same time it places the approximation in the X-register. To determine the accuracy of an approximation, check its uncertainty by pressing ®.

Example: With the display format set to i 2, calculate the integral in the expression for J1(1) (from the example on page 197).

Keystrokes

0 v

$

R

´i2

´f1

Display

 

0.0000

Key lower limit into

 

Y-register.

3.1416

Key upper limit into

 

X-register.

3.1416

(If not already in Radians mode.)

3.1400 Set display format to i2.1.300 Integral approximated in i2.

8

® 1.8 - Uncertainty of i2803 approximation.

The integral is 1.38 ± 0.00188. Since the uncertainty would not affect the approximation until its third decimal place, you can consider all the displayed digits in this approximation to be accurate. In general, though, it is difficult to anticipate how many digits in an approximation will be unaffected by its uncertainty. This depends on the particular function being integrated, the limits of integration, and the display format.

*No algorithm for numerical integration can compute the exact difference between its approximation and the actual integral. But the algorithm in the HP-15C estimates an ―upper bound‖ on this difference, which is the uncertainty of the approximation. For example, if the integral Si (2) is 1.6054 ± 0.0001, the approximation to the integral is 1.6054 and its uncertainty is 0.0001. This means that while we don't know the exact difference between the actual integral and its approximation, we do know that it is highly unlikely that the difference is bigger than 0.0001. (Note the first footnote on page 200.)