Here it is
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In nitrate ion due to resonance all nitrogen Oxygen bonds are of exact same type of bond length you can check the resonance hybrid in the given image of nitrate ion
In the image of nitrate ion   the resonance hybrid has only one type of bond length
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In nitric acid resonating structures ,one  oxygen is with hydrogen and other two oxygens are without hydrogen so there are different types of oxygen atoms .That is the reason we have two different types of bond lengths as it is clearly visible from the image of resonating structure of nitric acid.
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