Q 11 :

Let α,β,γ and δ be the coefficients of x7,x5,x3 ands x respectively in the expansion of (x+x31)5+(xx31)5, x>1. If u and v satisfy the equations αu+βv=18, γu+δv=20, then u + v equals :          [2025]

  • 5

     

  • 3

     

  • 4

     

  • 8

     

(1)

We have, (x+x31)5+(xx31)5

      = 2{C05x5+C25x3(x31)+C45x(x31)2}

                                 [(x+a)n+(xa)n=2(C0nxn+C2nxn2a2+...)]

      =2{5x7+10x6+x510x410x3+5x}

On comparing, we get

          α=10, β=2, γ=20, δ=10

Now, 10u + 2v = 18 and – 20u +10v = 20

 u = 1, v = 4

Hence, u + v = 5.



Q 12 :

The remainder when ((64)(64))(64) is divided by 7 is equal to          [2025]

  • 3

     

  • 1

     

  • 6

     

  • 4

     

(2)

Let A=((64)(64))(64)=(64)642

       A=(1+63)642, let 642=n

Expanding by binomial

       A=(1+63)n=1+C1n(63)+C2n(63)2+...=1+63λ=1+7(9λ)

   When ((64)(64))(64) is divisible by 7 then remainder is 1.



Q 13 :

If in the expansion of (1+x)p(1x)q, the coefficients of x and x2 are 1 and –2, respectively, then p2+q2 is equal to :           [2025]

  • 18

     

  • 8

     

  • 20

     

  • 13

     

(4)

(1+x)p(1x)q=(C0p+C1x+C2px2+...)(C0q-C1qx+C2px2+...)

Coeff. of xC0pC1q+C1pC0q=1

 pq=1          ... (i)

Coeff. of x2C0pC2qC1pC1q+C2pC0q=2

 q(q1)2pq+p(p1)2=2

 q2q2pq+p2p=4

 1(p+q)=4          [ q2+p22pq=(pq)2]

 p+q=5        ... (ii)

On solving equation (i) and (ii), we get p = 3 and q = 2.

So, .p2+q2=13



Q 14 :

Suppose A and B are the coefficients of 30th and 12th terms respectively in the binomial expansion of (1+x2)2n1. If 2A = 5B, then n is equal to:          [2025]

  • 22

     

  • 19

     

  • 21

     

  • 20

     

(3)

Given : A = Coefficient of 30th term in (1+x2)2n1

            B = Coefficient of 12th term in (1+x2)2n1

They can also be written as

           A=C292n1 and B=C112n1

Since, 2A = 5B, then

          2·C292n1 = 5·C112n1

  2·(2n1)!29!(2n30)! = 5·(2n1)!11!(2n12)!

  130×29×28×...×12

        =1(2n12)(2n13)(2n14)...(2n29)×12

On comparing, we get

          2n – 12 = 30  n = 21.



Q 15 :

The remainder, when 7103 is divided by 23, is equal to:          [2025]

  • 6

     

  • 14

     

  • 9

     

  • 17

     

(2)

We have, 7103=7(7102)=7(343)34=7(3452)34

=7(2)34=7(2)34=7·22·232=28·(256)4=28(253+3)4

=28(3)4=28×81=(23+5)(69+12)=(5)(12)=60

   Remainder = 14.



Q 16 :

The sum of the series 2×1×C4203×2×C520+4×3×C6205×4×C720+...+18×17×C2020, is equal to __________.          [2025]



(34)

Let 2×1×C4203×2×C520+...+18×17×C2020=A

Using binomial theorem,

(1+x)20=C020+C120x+C220x2+C320x3+...+C2020x20

(1+x)20x2=1x2+20x+C220+C320x+...+C2020x18

Differentiate with respect to x, we get

   2x(1+x)19(9x1)x4=2x320x2+C320+2·C420x+...+18C2020x17

   2(1+x)19(9x1)x3=2x320x2+C320+2·C420x+...+18C2020x17

Again, differentiate with respect to x, we get

   2[x3(1+x)19·9+x3(9x1)·19(1+x)18(1+x)19(9x1)·3x2]x6

=6x4+40x3+2·1·C420+3·2·C520x+...+18·17·C2020x16

Put x = –1 in above equation,

      0 = 6 – 40 + A

 A = 34.



Q 17 :

The product of the last two digits of (1919)1919 is __________.          [2025]



(63)

We have, (1919)1919=(19201)1919

        =C01919(1920)1919C11919(1920)1918+...+C19181919(1920)C19191919

        =(1920)19191919(1920)1918+...+1919(1920)1

        =100λ+1919(1920)1

                    [As all terms will have 100 as multiple except the last two terms]

        =100λ+36844801=100λ+3684479

   Last two digits will be 79

So, required product = 7×9=63.



Q 18 :

The sum of all possible values of nN, so that the coefficients of x,x2 and x3 in the expansion of (1+x2)2(1+x)n, are in arithmetic progression, is:           [2026]

  • 3

     

  • 7

     

  • 9

     

  • 12

     

(3)

(x4+2x2+1)(C0nx0+C1nx1+C2nx2+C3nx3+)

Coefficient of xC1n

Coeff. of x22+C2n=2+n(n-1)2

Coeff. of x3=2·C1n+C3n

=2n+n(n-1)(n-2)6  (if x3)

Now according to question

n+2n+n(n-1)(n-2)6=2[2+n(n-1)2]

3n+n(n-1)(n-2)6=4+n(n-1)

n3-9n2+26n-24=0

n=2,3,4  n=3,4

Now checking for n=2

Coeff. of x=2Coeff. of x2=3Coeff. of x3=4}are in A.P.

n=2 is also the correct choice

Required sum of values of n

=2+3+4=9

Option (3)



Q 19 :

The value of  C5010051+C5110052++C100100101  is:           [2026]

  • 2100101

     

  • 2100100

     

  • 2101101

     

  • 2101100

     

(1)

S=r=50100Cr100r+1=r=501001r+1·r+1101.101Cr+1

S=1101r=50100Cr+1101

=1101×21012=2100101

Option (1)



Q 20 :

Let S=125!+13!23!+15!21!+up to 13 terms. If 13S=2kn!, k, then n+k is equal to            [2026]

  • 50

     

  • 49

     

  • 52

     

  • 51

     

(2)

126!(26!25!1!+26!3!23!+26!5!21!++13 terms)

=126!(C126+C326+C526++13 terms)

=126!(C126+C326+C526++C2526)

S=126!×225

13S=22425!

so n+k=25+24=49



Q 21 :

The coefficient of x48 in (1+x)+2(1+x)2+3(1+x)3++100(1+x)100 is equal to             [2026]

  • 100·C49100-C48100

     

  • C50100+C49101

     

  • 100·C49101-C50101

     

  • 100·C49100-C50100

     

(3)

Let 1+x=r

S=1·r+2·r2+3·r3++100r100    ...(1)
                                                                              (AGP)

rS=1·r2+2·r3++99r100+100r101    ...(2)

(1)-(2) gives

S=-(1+x)101x2+1x2+100(1+x)101x

coefficient of x48 in S

=-coefficient of x48 in (1+x)101x2+100.Coefficient of x48 in (1+x)101x

=100.C49101-C50101



Q 22 :

The sum of the coefficients of x499 and x500 in (1+x)1000+x(1+x)999+x2(1+x)998++x1000 is:   [2026]

  • C5001002

     

  • C5011000

     

  • C5011001

     

  • C5011002

     

(1)

S=(1+x)1000+x(1+x)999+x2(1+x)998++x1000

=(1+x)1000(1-(x1+x)10011-x1+x)

=(1+x)1001-x1001

Required sum=C4991001 +C5001001=C5001002



Q 23 :

Given below are two statements :

Statement I : 2513+2013+813+313 is divisible by 7.

Statement II :The integral part of (7+43)25 is an odd number.

In the light of the above statements, choose the correct answer from the options given below :    [2026]

  • Both Statement I and Statement II are true

     

  • Statement I is false but Statement II is true

     

  • Both Statement I and Statement II are false

     

  • Statement I is true but Statement II is false

     

(1)

Statement I :

Statement II: R=(7+43)25=I+f

R'=(7-43)25=f'

 R+R'=2[C025725+C225723(43)2+]

I+f+f'=even integer

 I=odd integer

 0<f+f'<2f+f'=1

Both the statements are correct



Q 24 :

If (1C015+1C115)(1C115+1C215)(1C1215+1C1315)=α13C01414C114C12, then 30α is equal to_____ [2026]



(32)

r=012(1Cr15+1Cr+115)=r=01216r+1·Cr1515Cr·Cr+115

=r=01216(r+1)·15r+1·Cr14=r=0121615Cr14

=(1615)13C014·C114C1214α=1615

30α=32



Q 25 :

Let Cr denote the coefficient of xr in the binomial expansion of (1+x)nn 0rn.

If Pn=C0-C1+223C2-234C3++(-2)nn+1Cn, then the value of n=1251P2n equals:    [2026]

  • 675

     

  • 525

     

  • 650

     

  • 580

     

(1)

Pn=r=0nCrn(-2)rr+1=r=0n1n+1Cr+1n+1(-2)r

=-12(n+1)r=0nCr+1n+1(-2)r+1

=-12(n+1)[(1-2)n+1-1]

Pn=12(n+1)[1-(-1)n+1]

P2n=12(2n+1)[1-(-1)2n+1]

P2n=12n+1

n=1251P2n=n=125(2n+1)

=3+5++51

=252(51+3)

=25×27=675