why glucose and fructose both are giving possitive tests with Tollen's test?

Because glucose is having aldehyde group and fructuse in the form of ring chained structure have hemiacetal group....

So as both are giving positive tollen's test....

Generally aldehydes only gives Tollen's test. Thats why glucose only gives posssitive test with Tollen's reagent. But we know that Tollen's reagent is alkaline in nature, due to this when we add Tollen's reagent to Fructose it will converts in to glucose. Due to this conversion Fructose gives possitive test with Tollen's reagent. In this reaction glucose and fructose acts as the reducing agent.

glucose always gives +ve tollens test since it has a -CHO gp in its structure...but fructose,even though it has a keto gp, undergoes lobry de bruyn van eikenstein rearrangement and gives an equilibrium mixture of fructose, glocose and mannose where the last 2 give tollens test +ve.