JEE MAIN - Mathematics (2021 - 1st September Evening Shift - No. 10)
The numbers of pairs (a, b) of real numbers, such that whenever $$\alpha$$ is a root of the equation x2 + ax + b = 0, $$\alpha$$2 $$-$$ 2 is also a root of this equation, is :
6
2
4
8
Explanation
Consider the equation x2 + ax + b = 0
If has two roots (not necessarily real $$\alpha$$ & $$\beta$$)
Either $$\alpha$$ = $$\beta$$ or $$\alpha$$ $$\ne$$ $$\beta$$
Case (1) If $$\alpha$$ = $$\beta$$, then it is repeated root. Given that $$\alpha$$2 $$-$$ 2 is also a root
So, $$\alpha$$ = $$\alpha$$2 $$-$$ 2 $$\Rightarrow$$ ($$\alpha$$ + 1)($$\alpha$$ $$-$$ 2) = 0
$$\Rightarrow$$ $$\alpha$$ = $$-$$1 or $$\alpha$$ = 2
When $$\alpha$$ = $$-$$1 then (a, b) = (2, 1)
$$\alpha$$ = 2 then (a, b) = ($$-$$4, 4)
Case (2) If $$\alpha$$ $$\ne$$ $$\beta$$
Then
(I) $$\alpha$$ = $$\alpha$$2 $$-$$ 2 and $$\beta$$ = $$\beta$$2 $$-$$ 2
Hence, (a, b) = ($$-$$($$\alpha$$ + $$\beta$$), $$\alpha$$$$\beta$$)
($$-$$1, $$-$$2)
(II) $$\alpha$$ = $$\beta$$2 $$-$$ 2 and $$\beta$$ = $$\alpha$$2 $$-$$ 2
Then $$\alpha$$ $$-$$ $$\beta$$ = $$\beta$$2 $$-$$ $$\alpha$$2 = ($$\beta$$ $$-$$ $$\alpha$$) ($$\beta$$ + $$\alpha$$)
Since $$\alpha$$ $$\ne$$ $$\beta$$ we get $$\alpha$$ + $$\beta$$ = $$\beta$$2 + $$\alpha$$2 $$-$$ 4
$$\alpha$$ + $$\beta$$ = ($$\alpha$$ + $$\beta$$)2 $$-$$ 2$$\alpha$$$$\beta$$ $$-$$ 4
Thus $$-$$1 = 1 $$-$$2 $$\alpha$$$$\beta$$ $$-$$ 4 which implies
$$\alpha$$$$\beta$$ = $$-$$1 Therefore (a, b) = ($$-$$($$\alpha$$ + $$\beta$$), $$\alpha$$$$\beta$$)
= (1, $$-$$1)
(III) $$\alpha$$ = $$\alpha$$2 $$-$$ 2 = $$\beta$$2 $$-$$ 2 and $$\alpha$$ $$\ne$$ $$\beta$$
$$\Rightarrow$$ $$\alpha$$ = $$-$$ $$\beta$$
Thus $$\alpha$$ = 2, $$\beta$$ = $$-$$2
$$\alpha$$ = $$-$$1, $$\beta$$ = 1
Therefore (a, b) = (0, $$-$$4) & (0, 1)
(IV) $$\beta$$ = $$\alpha$$2 $$-$$ 2 = $$\beta$$2 $$-$$ 2 and $$\alpha$$ $$\ne$$ $$\beta$$ is same as (III) Therefore we get 6 pairs of (a, b)
Which are (2, 1), ($$-$$4, 4), ($$-$$1, $$-$$2), (1, $$-$$1), (0, $$-$$4)
Option (a)
If has two roots (not necessarily real $$\alpha$$ & $$\beta$$)
Either $$\alpha$$ = $$\beta$$ or $$\alpha$$ $$\ne$$ $$\beta$$
Case (1) If $$\alpha$$ = $$\beta$$, then it is repeated root. Given that $$\alpha$$2 $$-$$ 2 is also a root
So, $$\alpha$$ = $$\alpha$$2 $$-$$ 2 $$\Rightarrow$$ ($$\alpha$$ + 1)($$\alpha$$ $$-$$ 2) = 0
$$\Rightarrow$$ $$\alpha$$ = $$-$$1 or $$\alpha$$ = 2
When $$\alpha$$ = $$-$$1 then (a, b) = (2, 1)
$$\alpha$$ = 2 then (a, b) = ($$-$$4, 4)
Case (2) If $$\alpha$$ $$\ne$$ $$\beta$$
Then
(I) $$\alpha$$ = $$\alpha$$2 $$-$$ 2 and $$\beta$$ = $$\beta$$2 $$-$$ 2
Hence, (a, b) = ($$-$$($$\alpha$$ + $$\beta$$), $$\alpha$$$$\beta$$)
($$-$$1, $$-$$2)
(II) $$\alpha$$ = $$\beta$$2 $$-$$ 2 and $$\beta$$ = $$\alpha$$2 $$-$$ 2
Then $$\alpha$$ $$-$$ $$\beta$$ = $$\beta$$2 $$-$$ $$\alpha$$2 = ($$\beta$$ $$-$$ $$\alpha$$) ($$\beta$$ + $$\alpha$$)
Since $$\alpha$$ $$\ne$$ $$\beta$$ we get $$\alpha$$ + $$\beta$$ = $$\beta$$2 + $$\alpha$$2 $$-$$ 4
$$\alpha$$ + $$\beta$$ = ($$\alpha$$ + $$\beta$$)2 $$-$$ 2$$\alpha$$$$\beta$$ $$-$$ 4
Thus $$-$$1 = 1 $$-$$2 $$\alpha$$$$\beta$$ $$-$$ 4 which implies
$$\alpha$$$$\beta$$ = $$-$$1 Therefore (a, b) = ($$-$$($$\alpha$$ + $$\beta$$), $$\alpha$$$$\beta$$)
= (1, $$-$$1)
(III) $$\alpha$$ = $$\alpha$$2 $$-$$ 2 = $$\beta$$2 $$-$$ 2 and $$\alpha$$ $$\ne$$ $$\beta$$
$$\Rightarrow$$ $$\alpha$$ = $$-$$ $$\beta$$
Thus $$\alpha$$ = 2, $$\beta$$ = $$-$$2
$$\alpha$$ = $$-$$1, $$\beta$$ = 1
Therefore (a, b) = (0, $$-$$4) & (0, 1)
(IV) $$\beta$$ = $$\alpha$$2 $$-$$ 2 = $$\beta$$2 $$-$$ 2 and $$\alpha$$ $$\ne$$ $$\beta$$ is same as (III) Therefore we get 6 pairs of (a, b)
Which are (2, 1), ($$-$$4, 4), ($$-$$1, $$-$$2), (1, $$-$$1), (0, $$-$$4)
Option (a)
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