a body is movingwith variable acceleration (a) along a straight line. The avg acceleration of body in time interval t1 to t2 is?

1) a (t2+t1) /2

2) a (t2 -t1) /2

3) integration of a; limit t1 to t2 into dt/ t2+t1

4)integration of a; limit t1 to t2 into dt/ t2-t1.

Pls help experts.

Note: in 3 and4 option initial limit t1 and final limit t2.