Texas Instruments TI-92 Owners Manual - Page 86

Specifying Domain Constraints Many identities and transformations are valid for only a particular domain. For example: ln(xù y) = ln(x) + ln(y) only if x and/or y is not negative sinê (sin(q)) = q only if q , ë p/2 and q  p/2 radians Tip: Enter ln(xù y) instead of ln(xy); otherwise, xy is interpreted as a single variable named xy. Tip: For , or , press ¥ à or ¥ Â. You can also use 2 I 8 or 2 ¿ 2 to select them from a menu. Use the "with" operator to specify the domain constraint. Because ln(xùy) = ln(x) + ln(y) is not always valid, the logarithms are not combined. With a constraint, the identity is valid and the expression is simplified. Because sinê(sin(q)) = q is not always valid, the expression is not simplified. With a constraint, the expression can be simplified. Using Substitutions vs. Defining a Variable In many cases, you can achieve the same effect as a substitution by defining the variable. However, substitution is preferable for most cases because the variable is defined only for the current calculation and does not accidentally affect later calculations. Substituting x=1 does not affect the next calculation. Caution: After x is defined, it can affect all calculations that involve x (until you delete x). Storing 1!x affects the subsequent calculations. Chapter 3: Symbolic Manipulation 69