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10.3.3 “Unsafe” Simplifications

Calc is capable of performing some simplifications which may sometimes be desired but which are not “safe” in all cases. The a e (calc-simplify-extended) [esimplify] command applies the algebraic simplifications as well as these extended, or “unsafe”, simplifications. Use this only if you know the values in your formula lie in the restricted ranges for which these simplifications are valid. You can use Extended Simplification mode (m E) to have these simplifications done automatically.

The symbolic integrator uses these extended simplifications; one effect of this is that the integrator’s results must be used with caution. Where an integral table will often attach conditions like “for positive ‘a’ only,” Calc (like most other symbolic integration programs) will simply produce an unqualified result.

Because a e’s simplifications are unsafe, it is sometimes better to type C-u -3 a v, which does extended simplification only on the top level of the formula without affecting the sub-formulas. In fact, C-u -3 j v allows you to target extended simplification to any specific part of a formula.

The variable ExtSimpRules contains rewrites to be applied when the extended simplifications are used. These are applied in addition to EvalRules and AlgSimpRules. (The a r AlgSimpRules step described above is simply followed by an a r ExtSimpRules step.)

Following is a complete list of the “unsafe” simplifications.

Inverse trigonometric or hyperbolic functions, called with their corresponding non-inverse functions as arguments, are simplified. For example, ‘arcsin(sin(x))’ changes to ‘x’. Also, ‘arcsin(cos(x))’ and ‘arccos(sin(x))’ both change to ‘pi/2 - x’. These simplifications are unsafe because they are valid only for values of ‘x’ in a certain range; outside that range, values are folded down to the 360-degree range that the inverse trigonometric functions always produce.

Powers of powers ‘(x^a)^b’ are simplified to ‘x^(a b)’ for all ‘a’ and ‘b’. These results will be valid only in a restricted range of ‘x’; for example, in ‘(x^2)^1:2’ the powers cancel to get ‘x’, which is valid for positive values of ‘x’ but not for negative or complex values.

Similarly, ‘sqrt(x^a)’ and ‘sqrt(x)^a’ are both simplified (possibly unsafely) to ‘x^(a/2)’.

Forms like ‘sqrt(1 - sin(x)^2)’ are simplified to, e.g., ‘cos(x)’. Calc has identities of this sort for sin, cos, tan, sinh, and cosh.

Arguments of square roots are partially factored to look for squared terms that can be extracted. For example, ‘sqrt(a^2 b^3 + a^3 b^2)’ simplifies to ‘a b sqrt(a+b)’.

The simplifications of ‘ln(exp(x))’, ‘ln(e^x)’, and ‘log10(10^x)’ to ‘x’ are also unsafe because of problems with principal values (although these simplifications are safe if ‘x’ is known to be real).

Common factors are canceled from products on both sides of an equation, even if those factors may be zero: ‘a x / b x’ to ‘a / b’. Such factors are never canceled from inequalities: Even the extended simplifications are not bold enough to reduce ‘a x < b x’ to ‘a < b’ (or ‘a > b’, depending on whether you believe ‘x’ is positive or negative). The a M / command can be used to divide a factor out of both sides of an inequality.

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