cho ba điện tích q1=q2=q3=q đặt tại 3 đỉnh của hình vuông. Nếu lực tương tác giữa q1 và q2 là F12 và giữa q1 và q3 là F13 thì tỉ số giữa F12/F13 là gì

4

explanation:

4

a and b sometimes have a positive value

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me