JEE MAIN - Mathematics (2020 - 7th January Evening Slot - No. 18)

Let $$\alpha $$ and $$\beta $$ be the roots of the equation x2 - x - 1 = 0.
If pk = $${\left( \alpha \right)^k} + {\left( \beta \right)^k}$$ , k $$ \ge $$ 1, then which one of the following statements is not true?
(p1 + p2 + p3 + p4 + p5) = 26
p5 = 11
p3 = p5 – p4
p5 = p2 · p3

Explanation

x2 - x - 1 = 0

$$ \therefore $$ $$\alpha $$2 - $$\alpha $$ - 1 = 0

$$ \Rightarrow $$ $$\alpha $$2 = $$\alpha $$ + 1

$$ \therefore $$ $$\alpha $$3 = $$\alpha $$2 + $$\alpha $$

= $$\alpha $$ + 1 + $$\alpha $$

= 2$$\alpha $$ + 1

Now $$\alpha $$4 = 2$$\alpha $$2 + $$\alpha $$

= 2($$\alpha $$ + 1) + $$\alpha $$

= 3$$\alpha $$ + 2

Now $$\alpha $$5 = 3$$\alpha $$2 + 2$$\alpha $$

= 3($$\alpha $$ + 1) + 2$$\alpha $$

= 5$$\alpha $$ + 3

Given pk = $${\left( \alpha \right)^k} + {\left( \beta \right)^k}$$

$$ \therefore $$ p5 = $${\left( \alpha \right)^5} + {\left( \beta \right)^5}$$

= 5$$\alpha $$ + 3 + 5$$\beta $$ + 3

= 5($$\alpha $$ + $$\beta $$) + 6

= 5(1) + 6 [As $$\alpha $$ + $$\beta $$ = 1]

= 11

Now p2 · p3

= ($$\alpha $$2 + $$\beta $$2).($$\alpha $$3 + $$\beta $$3)

= ( $$\alpha $$ + 1 + $$\beta $$ + 1)(2$$\alpha $$ + 1 + 2$$\beta $$ + 1)

= ( $$\alpha $$ + $$\beta $$ + 2)(2($$\alpha $$ + $$\beta $$) + 2)

= (1 + 2)(2 + 2) = 12

$$ \therefore $$ p5 $$ \ne $$ p2 ·p3

So option (D) is wrong.

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