HP 40gs hp 40gs_user's guide_English_E_HDPMSG40E07A.pdf - Page 186

hp40g+.book Page 4 Friday, December 9, 2005 1:03 AM 10.Press three times to select the entire expression and then press to simplify it to the form required. CAS variables When you use the symbolic calculation functions, you are working with symbolic variables (variables that do not contain a permanent value). In the HOME screen, a variable of this kind must have a name like S1...S5, s1...s5, n1...n5, but not X, which is assigned to a real value. (By default, X is assigned to 0). To store symbolic expressions, you must use the variables E0, E1...E9. In the Equation Writer, all the variables may, or may not be, assigned. For example, X is not assigned to a real value by default, so computing X + X will return 2X. Moreover, Equation Writer variables can have long names, like XY or ABC, unlike in HOME where implied multiplication is assumed. (For example ABC is interpreted as A × B × C in HOME.) For these reasons, variables used in the Equation Writer cannot be used in HOME, and vice versa. Using the PUSH command, you can transfer expressions from the HOME screen history to CAS history (see page 14-8). Likewise, you can use the POP command to transfer expressions from CAS history to the HOME screen history (see page 14-8). The current variable In the Equation Writer, the current variable is the name of the symbolic variable contained in VX. It is almost always X. (The current variable is always S1 in HOME.) Some CAS functions depend on a current variable; for example, the function DERVX calculates the derivative with respect to the current variable. Hence in the Equation Writer, DERVX(2*X+Y) returns 2 if VX = X, but 1 if VX = Y. However, in the HOME screen, DERVX(2*S1+S2) returns 2, but DERIV(2*S1+S2,S2) returns 1. 14-4 Computer Algebra System (CAS)