Open raster _ extract pixel value

Hi, 
I begin to work with R software to treat raster data. 
Now, I have a NDVI image with a tiff extension. I want to open this image
and to extract NDVI value for some pixel which I have their coordinates. 
1-	Please what is the code to open a tiff image with R ?
2-	How to extract the value of some pixel which I have their coordinate ? 
NB: I hope you understand my questions with my bad English. 
Thank you 


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