Question

the tree grew at a dangerous slant and had to be cut ----------------------

Answer 1

The tree grew at a dangerous slant and had to be cut down
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for this we can use both 'off ' and 'down'

but more appropriate one will be down

Answer 2

Hope this helps you...
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The tree grew at a dangerous slant and had to be cut down
Or
The tree grew at a dangerous slant and had to be cut off
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for this we can use both 'off' and "down"
Use that you feel appropiate . For me down is more appropiate one.