technical analysts believe that investors can use past price changes to predict future price changes. how do they justify this belief?

You make an incorrect statement, then ask a question about it, in effect, slandering an entire profession.

Nobody, not one single person, ever, has been able to "predict" future price any better than a coin toss. That is not what we do as traders/investors. We are risk managers, not fortune tellers. We rely on our risk model and trade plan, and tested probability of success, not chance or opinion or false belief or rolling the dice and guessing (like most other market players).

We use Technical Analysis (TA) to measure price moves and risk, and TA is one way to lower risk. Sometimes we use a projection (not prediction) for extended moves or support levels, but most people don't know the difference and therefore make wrong presumptions.

Crowd psychology dictates that people will act similarly in times of greed and fear. TA attempts to measure that, not predict anything. A trader has a plan for both directions, not just one. Successful traders have a keen sixth sense, but still no predictive capability (regardless of whether they use TA -- TA is just a tool, like a computer or a hammer). They seem to understand real risk and this is why genius fails but market making doesn’t, for example.

IMO Short term trends can be determined sometimes by looking at a graph. Otherwise they're probably right 51% of the time. I think the indicators are cool, MACD, Bollinger Bands, CMF, it tells you what people are doing. You have to know volume, volitility, etc. But people who predict a future prices by circling 2 camel humps and setting a resistance point seems like BS. One thing that can move price is traders buying or selling, and if a certain amount of traders believe this stuff, it will actually be correct. Not because you circle camel humps and head and shoulders, but from the reaction of the action of doing so.

I have no evidence to back this up.

If you take AAPL, knowing their earnings, and knowing when their iwatch comes out is more important than any chart. If you mean day trading, I don't know. But for predicting future prices for investing, financial analysis and the news are better. Along with charting.

Every time you invest money, you're taking a risk. Your own carefully conducted research should give you an idea of how much risk you are taking.

When you buy a stock, "past performance has no bearing on future performance" - this (or a statement very like it) will be somewhere in the small-print. DON'T EVER FORGET THAT! Anyone who tells you otherwise is lying or trying to con you out of money.

Technical Analysts might look at past performance for specific patterns of behaviour, but they also make use of external information to form an analysis and a conjecture.

Looking at trends is just one of the tools they use.

Sometimes it works, sometimes it fails.

they use the business models to demonstrate the coorelation