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The a d (calc-derivative
) [deriv
] command computes
the derivative of the expression on the top of the stack with respect to
some variable, which it will prompt you to enter. Normally, variables
in the formula other than the specified differentiation variable are
considered constant, i.e., `deriv(y,x)' is reduced to zero. With
the Hyperbolic flag, the tderiv
(total derivative) operation is used
instead, in which derivatives of variables are not reduced to zero
unless those variables are known to be "constant," i.e., independent
of any other variables. (The built-in special variables like pi
are considered constant, as are variables that have been declared
const
; see section 7.6 Declarations.)
With a numeric prefix argument n, this command computes the nth derivative.
When working with trigonometric functions, it is best to switch to radians mode first (with m r). The derivative of `sin(x)' in degrees is `(pi/180) cos(x)', probably not the expected answer!
If you use the deriv
function directly in an algebraic formula,
you can write `deriv(f,x,x0)' which represents the derivative
of f with respect to x, evaluated at the point
x=x0.
If the formula being differentiated contains functions which Calc does
not know, the derivatives of those functions are produced by adding
primes (apostrophe characters). For example, `deriv(f(2x), x)'
produces `2 f'(2 x)', where the function f'
represents the
derivative of f
.
For functions you have defined with the Z F command, Calc expands
the functions according to their defining formulas unless you have
also defined f'
suitably. For example, suppose we define
`sinc(x) = sin(x)/x' using Z F. If we then differentiate
the formula `sinc(2 x)', the formula will be expanded to
`sin(2 x) / (2 x)' and differentiated. However, if we also
define `sinc'(x) = dsinc(x)', say, then Calc will write the
result as `2 dsinc(2 x)'. See section 18.4 Programming with Formulas.
For multi-argument functions `f(x,y,z)', the derivative with respect
to the first argument is written `f'(x,y,z)'; derivatives with
respect to the other arguments are `f'2(x,y,z)' and `f'3(x,y,z)'.
Various higher-order derivatives can be formed in the obvious way, e.g.,
`f'''(x) (the second derivative of f
) or
`f'''2'3(x,y,z) (f
differentiated with respect to each
argument once).
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