1.

Number of positive integers `x`for which `f(x)=x^3-8x^2+20 x-13`is a prime number is______.

Answer» Clearly, `f(1) = 0`
`therefore f(x) = (x - 1) (x^(2) - 7x + 13)`
For f(x) to be prime, at least one of the factors must be prime. Hence, either x - 1 = 1
or `x^(2) - 7x + 13 = 1`
If x - 1 = 1, then x = 2
If `x^(2) - 7x + 13 = 1 ,` then` x^(2) - 7x + 12 = 0`
`rArr x = 3 or 4`
So, x = 2, 3, 4


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