Below is the velocity-time graph for a body moving with an initial velocity u and uniform acceleration a:
Let v be the velocity at time t for this body having initial velocity u and uniform acceleration a as shown in the graph.
Distance travelled by the body in time t = Area under the velocity-time curve.
Hence,
Distance travelled = area of trapezium OABD
= 1\over 2 Sum of parallel sides × height
= 1\over 2 × (OA + DB) × OD
= 1\over 2 × (u + v) × t
= (u+v)t\over 2