Substitue a variable by a value : subst

subst takes two or three arguments :

• an expression depending on a variable, an equality (variable=value of substitution) or a list of equalities.
• an expression depending on a variable, a variable or a list of variables, a value or a list of values for substitution.

subst returns the expression with the substitution done. Note that subst does not quote it's argument, hence in a normal evaluation process, the substitution variable should be purged otherwise it will be replaced by it's assigned value before substitution is done.
Input :

subst(a^2+1,a=2)

or :

subst(a^2+1,a,2)

Output (if the variable a is purged else first input purge(a)) :

5

Input :

subst(a^2+b,[a,b],[2,1])

Or :

subst(a^2+b,[a=2,b=1])

Output (if the variables a and b are purged else first input purge(a,b)) :

2^2+1

subst may also be used to make a change of variable in an integral. In this case the integrate command should be quoted (otherwise, the integral would be computed before substitution) or the inert form Int should be used. In both cases, the name of the integration variable must be given as argument of Int or integrate even you are integrating with respect to x.
Input :

subst('integrate(sin(x^2)*x,x,0,pi/2)',x=sqrt(t))

Or :

subst(Int(sin(x^2)*x,x,0,pi/2),x=sqrt(t))

Output

integrate(sin(t)*sqrt(t)*1/2*1/t*sqrt(t),t,0,(pi/2)^2)

Input :

subst('integrate(sin(x^2)*x,x)',x=sqrt(t))

Or :

subst(Int(sin(x^2)*x,x),x=sqrt(t))

Output

integrate(sin(t)*sqrt(t)*1/2*1/t*sqrt(t),t)