How to calculate the lenght of the segments a and b of a line c (a+b) using the golden proportion (1.618)?

Question

ie. How to divide a line c (where c=a+b) into two unequal parts: a and b

Answer 1

Your question is ambiguous. Perhaps you mean, how do I cut a line segment into two pieces whose ratio is the golden ratio?

The golden ratio is usually designated by φ.

φ = (√5 + 1)/2 ≈ 1.618

For a line of unit length, cut it into two segments of lengths
1/φ and 1/φ² ≈ 0.618 and 0.382

If the line is of length c, then the segment lengths are:
c/φ and c/φ²