SUBST
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Syntax
SUBST ( expression | equation , equation | equation list )
Implemented in iMath since version 2.2.0 or earlier.
Explanation
Substitute the equation or the list of equations into the expression. All substitutions will be done in parallel, which means that after substituting a list of equations like x = y; y = z the expression might still contain the variable y. In this case you need to use SUBSTC instead.
Example
@eq:cubic@ EQDEF x^3 + a x^2 + b x + c = 0
@eq:tsch3@ EQDEF y = x + {a over 3}
EQDEF COLLECT(SIMPLIFY(SUBSTC(@eq:cubic@, SOLVE(@eq:tsch3@, x, 1)),"expand"), y)
This will perform the Tschirnhaus transformation on the cubic equation, reducing it to the depressed form y^3 + p y + q.