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Why do moving average acts as support and resistance?
A moving average will act as support or resistance to a stock only when the stock is trending. The way it acts as support for instance is similar to a trend-line. Take the daily chart of CBA over the last 6 months: The first chart shows CBA with an uptrend support line.  The second chart shows CBA during the same period with 50 day EMA as a support.  Both can be used as support for the uptrend. Generally you can used these types of support (or resistance in a downtrend) to determine when to buy a stock and when to sell a stock. If I was looking to buy CBA whilst it was uptrending, one strategy I could use was to wait until it hit or got very close to the support trend-line and then buy as it re-bounces back up. If I already held the stock I could use a break down below the uptrend support line as a stop to exit out of the stock.