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Find the least value of ' a ' such that ...

Find the least value of `' a '` such that the function `f(x)=x^2+a x+1` is increasing on `[1,\ 2]` . Also, find the greatest value of `' a '` for which `f(x)` is decreasing on `[1,\ 2]` .

A

`(-2,oo)`

B

`[-4,oo)`

C

`[-oo,-2)`

D

`(-oo,2]`

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A
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