Revision history [back]

Your question actually appears to be two homework questions.

1) Find the x-intercept for the given polynomial.

Your code has some extraneous lines in it. So, I'll shorten it.

f(x)=x^3 + 4*x - 7
p1= plot(f(x), (x,-15,15), ymin=-15, ymax=15)
show(p1)

This plot shows just one real root.

ans=solve(f(x)==0,x)
print ans

Solve gives two nonreal roots and one real root. This is consistent with the plot, and you can get the real root using:

ans[2].rhs().n()

Another option is to use find_root to find the root numerically:

find_root(f(x),0,3)

Either way, you get the answer you expected: 1.25538315684475

Substituting this into f should give you 0, and what you saw was that you got 10^(-15) which is VERY close to zero. This often happens when using numerical approximations.

2) given f(x) = x^3 + 4*x - 2, one needs to evaluate f^-1(-5)

You need to use solve find x-values for which f(x)=-5.