Let θ denote the angular displacement of a simple pendulum oscillating in a vertical plane. If the mass of the bob is m, the tension in the string is mg cosθ

Tension in a string at any position is given as , where r is the length of pendulum and v is the instantaneous velocity of the bob. Now at the extremes points velocity of the bob is zero. Hence the term nullifies leaving alone . So, the tension in the string at extreme points is .