Official Quant thread for CAT 2013

@chandrakant.k said:
sab goemetry ke piche pade ho ye loTen tickets are numbered 1,2,3.........10. six tickets are selected at random one at a time with replacement. what is the prob that largest no. appearing on the selected ticket is 7 is ??
(7^6-6^6)/10^6
@chandrakant.k ..haha kya baat hai bhai..itna gussa geometry pe..
@krum said:
(7^6-6^6)/10^6
bhai is ques ka logic zaroor dena ..ye ques to phle wale thread pe kafi chaya hua tha..
@nick_baba said:
bhai is ques ka logic zaroor dena ..ye ques to phle wale thread pe kafi chaya hua tha..
10^6 total
ab max 7 chahiye to 1-7 hi select kar sakte hain , so 7^6
par isme cases aaenge jisme 7 ek bhi bar select nai hua ho, aise max. cases tab honge jab 1-6 mein hi select kar lia ho - 6^6
so (7^6-6^6)/10^6
@gs4890 said:
@maddy2807@Brooklyn@sumeet1489@vijay_chandolaPFA Cat qstnFind ratio of - (area under red / area under blue)
these are squares and circles.
ration will be 4:pi:2

so, red/blue= 4-pi/pi-2= 75%
@nick_baba said:
@chandrakant.k ..haha kya baat hai bhai..itna gussa geometry pe..
The number of ways of selecting 1,2,3,4,5,6,7 = (7^6)
The number of ways in which any one of 1,2,3,4,5,6 is the highest number = (6^6)

Required ways = ( 7^6 - 6^6 )

Total ways = 10^6

Probability = (7^6 - 6^6)/10^6
P.S : Geometry se nafrat hai muje... bachpan se aaj tak...
@krum said:
10^6 totalab max 7 chahiye to 1-7 hi select kar sakte hain , so 7^6par isme cases aaenge jisme 7 ek bhi bar select nai hua ho, aise max. cases tab honge jab 1-6 mein hi select kar lia ho - 6^6so (7^6-6^6)/10^6

nai hua ho, aise max. cases tab honge jab 1-6 mein hi select kar lia ho - 6^6

Isko aur ek baar aur samjha diyo..
@bob_biswas said:
Is the answer c)R(n-1) = Rn/ ( 3+2*root(2)) So, infinite GP with r = 1/(3 + 2root(2)) -> Rn*(1 - r^n) / ( 1-r) n approaches infinity = > Rn*(1/(1-r)) => option C
kya baat hai.bhai..phla post aur bang on.......cheers..
@rushabhramani said:
nai hua ho, aise max. cases tab honge jab 1-6 mein hi select kar lia ho - 6^6Isko aur ek baar aur samjha diyo..
yar max. 7 chahiye hi
par 7^6 includes cases like 1,1,1,1,1,1; 6,6,6,6,6,6; 1,2,4,5,1,6;
i.e u are not selecting 7th, so condition is no fulfilled
remove all such cases where 7 is not included =>7^6-6^6
@krum thank you ji

@chandrakant.k ...lo bhai..apna gussa thoda shaant kar lo....algebra se khelte hai chalo..

Q.) a, b, c, d be real numbers in G.P. If u, v,w satisfy the equations u +
2v + 3w = 6; 4u + 5v + 6w = 12; 6u + 9v = 4 then roots of the equations
(1/u + 1/v + 1/w)x2 + [(b − c)2 + (c − a)2 + (d − b)2]x + u + v + w = 0 and
20x2 + [10(a − d)2]x − 9 = 0 are

a) equal to each other
b) reciprocal to each other
c) at least one of the foregoing
d) all of the foregoing
@bob_biswas said:
It is whole square right ? I mean the co-efficient 2 written inside brackets.Solving we get u = -1/3 , v = 2/3, w = 5/3 .. We see, that coefficient of x^2 of one is same as constant terms of the other and the middle term gets adjusted ... => reciprocal ? Option b) ?
well yes..they are powers..but which coef of x?^2 u r lukng at...and comparing with which const term...n hw u adjusted middle term????
@nick_baba said:
@chandrakant.k ...lo bhai..apna gussa thoda shaant kar lo....algebra se khelte hai chalo..Q.) a, b, c, d be real numbers in G.P. If u, v,w satisfy the equations u +2v + 3w = 6; 4u + 5v + 6w = 12; 6u + 9v = 4 then roots of the equations(1/u + 1/v + 1/w)x2 + [(b − c)2 + (c − a)2 + (d − b)2]x + u + v + w = 0 and20x2 + [10(a − d)2]x − 9 = 0 area) equal to each other b) reciprocal to each other c) at least one of the foregoing d) all of the foregoing
u=-1/3,v=2/3,w=5/3

(1/u + 1/v + 1/w)x2 + [(b − c)2 + (c − a)2 + (d − b)2]x + u + v + w = 0
reduces to

-9x^2+[10(a-d)^2]x+20=0
&
20x2 + [10(a − d)^2]x − 9 = 0

b) reciprocal to each other
@bob_biswas said:
Well, I didn't adjust the middle term but my sixth sense tells me that it will get adjusted I used this property : f(x) = ax^2 + bx + c = 0 Reciprocal roots will satisfy the quadratic, g(x) = c(x^2) + b(x) + a = 0So, you can see that the constant term and x^2 got interchanged.
@krum said:
u=-1/3,v=2/3,w=5/3(1/u + 1/v + 1/w)x2 + [(b − c)2 + (c − a)2 + (d − b)2]x + u + v + w = 0reduces to-9x^2+[10(a-d)^2]x+20=0&20x2 + [10(a − d)^2]x − 9 = 0b) reciprocal to each other
u bot r right..
@nick_baba said:
well yes..they are powers..but which coef of x?^2 u r lukng at...and comparing with which const term...n hw u adjusted middle term????
mera bhi same question hai .. middle term adjust nahi ho raha hai... baki sab hogaya...
@krum bhai how to go with middle term?
@chandrakant.k said:
mera bhi same question hai .. middle term adjust nahi ho raha hai... baki sab hogaya... @krum bhai how to go with middle term?
there's a property for G.P--a,b,c,d are in GP..then (a-d)^2= [(b − c)^2 + (c − a)^2 + (d − b)^2]...
an easy one...

For how many primes p is p^2 + 3p ˆ' 1 also prime?
a) 0 b) 1 c) 2 d) 3

koi mast sa general sol batao..option dekh ke mat karna..
In a certain town, every girl is acquainted with more boys then girls.
Furthermore, all the girls acquainted with a given boy are also acquainted
with each other.Then

a) The number of boys is always greater than the number of girls.
b) The number of girls is always greater than the number of boys .
c) There are at least as many boys as girls in the town .
d) There are at least as many girls as boys in the town.



@nick_baba said:
an easy one...For how many primes p is p^2 + 3p − 1 also prime?a) 0 b) 1 c) 2 d) 3 koi mast sa general sol batao..option dekh ke mat karna..
every prime number except 2 and 3 are of the from 6k+1/6k-1
p^2 + 3p − 1
=>(6k+1)^2+18k+2
=>36k^2+30k+3

or
=>(6k-1)^2
+18k-4
=>36k^2+6k-3

as both are multiples of 3, only p=3 will give a prime number

b)1
@chandrakant.k said:
mera bhi same question hai .. middle term adjust nahi ho raha hai... baki sab hogaya... @krum bhai how to go with middle term?
i expanded [(b − c)^2 + (c − a)^2 + (d − b)^2]
=>b^2+c^2-2bc+c^2+a^2-2ac+d^2+b^2-2db

as a,b,c,d are in g.p =>b^2=ac, c^2=bd

=>b^2+c^2-2bc+c^2+a^2-2b^2+b^2+d^2-2c^2
=>a^2+d^2-2bc
=>a^2+a^2r^6-2ar^3
=>(a-ar^3)^2
=>(a-d)^2
@bob_biswas said:
b) 1 ?p^2 + 3p - 1 = N All primes other than 2,3 are of the form 6k+1/ 6k-1so, for p = 2, 3 , we need to check manually -> p = 2 => p^2 + 3p - 1 = 9 (rejected)p = 3 => p^2 + 3p -1 = 17 Now for p = 6k+ 1 or 6k - 1N = (p-1)*(p+1) +3pThis N is divisible by 3 => No other value of p exists.
correctum...