Two rods, one of aluminium and the other made of steel, having initial length  l1 and l2  respectively are  connected together to  form a single rod of length l1+l2 . The coefficient  of linear  expansion for aluminium and steel are  αa and  αs respectively. If the length of each rod  increases by the same amount when their  temperature t0C are raised by  , then  find the ratio  l1l1+2