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First I would go a bit far from the tree so that when I take a ruler, or perhaps a pencil the height of the tree looks exactly as the height of the tree. then i would turn the pencil to 90degree so that now from the height position, the pencil would be on the length position.

I would ask my friend to stand at the end of the pencil. then I would take a measuring tape to measure the actual distance- from the tree to where my friend is standing, which would give me the height.