Solve bx + c = 0 for x by means of the quadratic formula.
Let's find the answer to ax^2+bx+c=0 when a goes to zero. The usual formula is (-b+-√(bē-4ac))/2a which equals b(-1+-√(1-4ac/bē))/2a.When z goes to zero, the square root of (1-z) is approximately 1-z/2, so our square root above can be approximated by 1-2ac/bē. Substituting this approximation, we get two possible values: infinity (meaningless here) and -c/b, which is the desired result.
By limits
