# Solve for all values of a variable

Hello,

solve(ax+b==cx+d,a,b)

Output: ([a == (c*x - b + d)/x], [1])

I need: [a==c, b==d] for all x.

Thanks!

edit retag close merge delete

Sort by ยป oldest newest most voted

If you want to match coefficients of like terms, you could do something like

from collections import defaultdict
def solve_matched_polynomials(eq, coeff_vars):
# get the non-coefficient variables
vv = sorted(set(eq.variables()).difference(coeff_vars))
# make sure they're polynomials
assert all(eq.lhs().is_polynomial(v) and eq.rhs().is_polynomial(v) for v in vv)
d = defaultdict(Integer)
# loop over each term in the equation
for term in (eq.lhs().operands())+(-(eq.rhs())).operands():
# get the degrees of the variables
vsig = tuple(term.degree(v) for v in vv)
# find the non-variable part
d[vsig] += term/(prod(v**vs for v,vs in zip(vv, vsig)))
# get solutions with free variables
return solns = solve(d.values(), coeff_vars)


which will return the standard free variables form

sage: solve_matched_polynomials(5*a*x^2*y+b*x+y == 4*c*x**2*y-d*y, [a,b,c,d])
[[a == 4/5*r66, b == 0, c == r66, d == -1]]


and then if you really wanted to eliminate the free variables, you could.

more