Solar panels are arrays of photovoltaic cells that convert solar radiation into electricity. the table shows the annual percent change in the US price per watt of a solar panel
Year 2004 2005 2006 2007 2008
% growth
over previous yr -5.7 6.7 9.7 -3.7 3.6
Find the largest time interval over which the actual price per watt of a solar panel was an increasing function of time.