Let the distance travelled by the body be “S”.We know thatDistance = Average velocity × TimeAlso, Average velocity =u+v2∴Distance = u+v2 × t ......................................(1)Again we know thatv = u + atsubstituting this value of “v” in eq.(1), we getDistance = u+u+at2 × tS = 2ut +at22S = ut + 12at2this is second equation of motion . Now let's derive third eqation of motionFrom the first equation of motion v = u + a t∴t =v−uaAverage velocity is given as = v+u2Distance = Average velocity × TimeS =v+u2 ×v−uaS = v2 − u22av2 = u2 + 2asThis is third equation of motion