explain the conditions necessary for a devaluation to improve the
balance of trade for absorption approach and elasticties approach?

For the elasticity approach, "devaluation will improve the trade
balance if domestic demand elasticity for imports plus foreign demand
elasticity for exports is greater than one.  Devaluation will worsen
the trade balance if the sum of the two elasticities is less than one.
 If the sum is equal to one, devaluation will have no effect."  This
is known as the Marshall-Lerner condition.

For the absorption approach, "devaluation will only improve the trade
balance if output rises relative to domestic absorption.  If an
economy is operating below capacity, a devaluation will shift
resources into export production and encourage spending on import
substitutes."  However, "if an economy is operating at full
employment, production cannot rise...."  Under that circumstance, the
trade imbalance will only be improved If the domestic economy slows,
thereby decreasing demand for imports.

Source: International Economics, by Robert J. Carbaugh, ninth edition,
South-Western College Publishing, 2004
http://www.bu.edu/goglobal/courses/intecon/ppslides/carbaugh9e-ch15.pps

Sincerely,

Wonko