How to use Reducer function in Google Earth Engine

In Tutorial i will present, how to use reducer function in google earth engine
Reducers take an input dataset and produce a single output. When a single input reducer is applied to a multi-band image, Earth Engine automatically replicates the reducer and applies it separately to each band. As a result, the output image has the same number of bands as the input image; each band in the output is the reduction of pixels from the corresponding band in the input data. Some reducers take tuples of input datasets. These reducers will not be automatically replicated for each band. For example, #ee.Reducer.LinearRegression() takes multiple predictor datasets (representing independent variables in the regression) in a particular order (see Regression reducers).
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