JEE mathematics

Sponsor Area

Question
CBSEENMA11015538

If, for a positive integer n, the quadratic equation,
x(x + 1) + (x + 1) (x + 2) + .....
+ (x + n -1 ) (x + n) = 10n
has two consecutive integral solutions, then n is equal to :

  • 11

  • 12

  • 9

  • 10

Solution

A.

11

nx squared space plus space straight x left parenthesis 1 space plus space 3 plus 5 plus..... space plus space left parenthesis 2 straight n minus 1 right parenthesis right parenthesis space plus
space left parenthesis 1.2 space plus space 2.3 space plus space..... space plus space left parenthesis straight n minus 1 right parenthesis. straight n right parenthesis minus 10 straight n space equals space 0
rightwards double arrow space nx squared space plus space straight x space left parenthesis straight n squared right parenthesis space plus space fraction numerator straight n left parenthesis straight n squared minus 1 right parenthesis over denominator 3 end fraction space minus space 10 straight n space equals space 0
rightwards double arrow space straight x squared space plus space straight x space left parenthesis straight n right parenthesis space plus space fraction numerator left parenthesis straight n squared minus 1 right parenthesis over denominator 3 end fraction minus 10 space equals space 0 space less than subscript straight beta superscript straight alpha
left parenthesis straight alpha space minus space straight beta right parenthesis squared space equals space 1
rightwards double arrow space left parenthesis straight alpha space plus space straight beta right parenthesis squared space minus space 4 αβ space equals space 1
rightwards double arrow space straight n squared space minus space 4 space open parentheses fraction numerator straight n squared minus 1 over denominator 3 end fraction minus 10 close parentheses space equals space 1
rightwards double arrow space straight n space equals space 11

Sponsor Area

Question
CBSEENMA11015539

The function straight f colon space straight R space rightwards arrow with space on top space open square brackets negative 1 half comma 1 half close square brackets space defined space as space straight f left parenthesis straight x right parenthesis space equals space fraction numerator straight x over denominator 1 plus space straight x squared end fraction comma space is

  • neither injective nor surjective.

  • invertible

  • injective but not surjective.

  • surjective but not injective

Solution

D.

surjective but not injective


straight f left parenthesis straight x right parenthesis space equals space fraction numerator straight x over denominator 1 space plus space straight x squared end fraction semicolon space straight f colon space straight R space rightwards arrow open square brackets negative 1 half comma 1 half close square brackets
From above diagram of f(x), f(x) is surjective but not injective.

Question
CBSEENMA11015540

The following statement
(p → q ) → [(~p → q) → q] is

  • a fallacy

  • a tautology

  • equivalent to ~ p → q

  • equivalent to p → ~q

Solution

B.

a tautology

(p → q) → [(~p → q) →q]
(p → q) → ((p → q) → q)
(p → q) → ((~p → ~q) → q)
(p → q) → ((~p→ q) → (~q→ q))
(p→ q) v (p → q) which is tautology

Question
CBSEENMA11015541

For any three positive real numbers a, b and c, (25a2 + b2) + 25(c2 – 3ac) = 15b(3a + c), Then

  • b , c and a are in G.P

  • b, c and a are in A.P

  • a, b and c are in A.P

  • a, b and c are in G.P

Solution

B.

b, c and a are in A.P

225a2 + 9b2 + 25c2 – 75ac – 45ab – 15bc = 0
(15a)2 + (3b)2 + (5c)2 – (15a)(3b) – (3b)(5c) – (15a) (5c) = 0
1/2[(15a – 3b)2 + (3b – 5c)2 + (5c – 15a)2] = 0
15a = 3b , 3b = 5c , 5c = 15a
5a = b , 3b = 5c , c = 3a
a/1 = b/5 = c/3
a = λ, b = 5λ, c = 3λ
a, c, b are in AP

Question
CBSEENMA11015542

hyperbola passes through the point P(√2, √3) and has foci at (± 2, 0). Then the tangent to this hyperbola at P also passes through the point

  • left parenthesis negative square root of 2 comma space minus square root of 3 right parenthesis
  • left parenthesis 3 square root of 2 space comma space 2 square root of 3 right parenthesis
  • left parenthesis 2 square root of 2 space comma 3 space square root of 3 right parenthesis
  • left parenthesis square root of 3 comma square root of 2 right parenthesis

Solution

C.

left parenthesis 2 square root of 2 space comma 3 space square root of 3 right parenthesis

Equation of hyperbola is straight x squared over straight a squared minus straight y squared over straight b squared space equals space 1
foci is (±2, 0) hence ae = 2, ⇒ a2e2 = 4

b2 = a2(e2 – 1)
∴ a2 + b2 = 4
Hyperbola passes through √2,√3
therefore space 2 over straight a squared space minus 3 over straight b squared space equals space 1 space... space left parenthesis 2 right parenthesis
On space sol space left parenthesis 1 right parenthesis space and space left parenthesis 2 right parenthesis
straight a squared space equals 8 space left parenthesis is space rejected right parenthesis space and space straight a squared space equals space 1 space and space straight b squared space equals 3
therefore space straight x squared over 1 minus straight y squared over 3 space equals 1
Equation space of space tangent space is space fraction numerator square root of 2 straight x over denominator 1 end fraction minus fraction numerator square root of 3 straight y over denominator 3 end fraction space equals 1
Hence space left parenthesis 2 square root of 2 comma 3 space square root of 3 right parenthesis space satisfy space it.