Is this conjecture always true?

If you bisect a line with a line extending from an angle oposite it is the angle split in the same ratio as the side? That question is somewhat confusing so say you have an equilateral triangle and a line from a vertex splits the opposite side into two segments, one three times the size of the other (1:3 length ratio). Would it be possible to say with certainty that the angle had been divided by that line into a three to one ratio i.e. (since equilateral triangles always have angle measures of 60 degrees) that the line had split it into two angles: one of 45 degrees and the other 15 degrees?