Una onda pasa por una frontera donde su velocidad varia de 6 km/s a 8,5 km/s. Si llega a la frontera formando un angulo de 40 grados... ¿cuál es el ángulo de refraccion?

this allows people to make the best decision or pursue the best option. when alternatives are weighed using their advantages and disadvantages, the option whose disadvantages outweigh the advantages (based on a qualitative or quantitative analysis) are shunned while those whose advantages outweigh their disadvantages are pursued.  

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.