According to the diagram, we know that if we apply a respectively force of 3N and 4N in the relative direction, then the resultant force will be 5N on its direction. But it's that means some of the energy lost to the surrounding? We applied a total force of 7N but the resultant force is only 5N, doesn't it prompt that there are some energy lost ?
First of all, the diagram has issues. Note how the angle is expressly $>90$ degrees and so the longest arrow is $>\sqrt{4^2 +3^2}$.
In words, the force vector labeled “5N” is not $5N$ of force. The only way this could happen is if the angle between the two vectors was exactly $90$ degrees.
As an aside, the “energy” to be considered here is likely work done which is $\int{\bf F}\cdot d{\bf r}$ but we can’t really discuss this in this context without knowing details about the path the particle is taking. We don’t really have a physical picture here, just a force diagram.
It doesn’t really make sense to consider energy in this context (as the question is posed).
