Question
If (1+x)n=C0+C1x+C2x2+........+Cnx2, then
C20+C21+C22+C23+..........+C2n =
C20+C21+C22+C23+..........+C2n =
Answer: Option B
:
B
(1+x)n=C0+C1x+C2x2+.........+CnXn ...........(i)
and (1+1x)n =C0+C11x+C2(1x)2+........+Cn(1x)n ..........(ii)
If we multiply (i) and (ii), we get
C20+C21+C22+........+C2n
is the term independent of x and hence it is equal to the term independent of x in the product (1+x)n(1+1x)n or in 1xn(1+x)2n or term
containing xn in (1+x)2n. Clearly the coefficient of xn in (1+x)2x is Tn+1 and equal to2nCn = (2n)!n!n!
Trick:Solving conversely.
Put n = 1, n = 2,.........then we get S1 = 1C20+1C21= 2,
S2 =2C20+2C21+2C22=12+22+12 = 6
Now check the options
(a) Does not hold given condition,
(b) (i) Put n = 1, then 2!1!1!=2
(ii) Put n = 2, then 4!2!2!=4×3×2×12×1×2×1=6
Note: Students should remember this question as identity
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:
B
(1+x)n=C0+C1x+C2x2+.........+CnXn ...........(i)
and (1+1x)n =C0+C11x+C2(1x)2+........+Cn(1x)n ..........(ii)
If we multiply (i) and (ii), we get
C20+C21+C22+........+C2n
is the term independent of x and hence it is equal to the term independent of x in the product (1+x)n(1+1x)n or in 1xn(1+x)2n or term
containing xn in (1+x)2n. Clearly the coefficient of xn in (1+x)2x is Tn+1 and equal to2nCn = (2n)!n!n!
Trick:Solving conversely.
Put n = 1, n = 2,.........then we get S1 = 1C20+1C21= 2,
S2 =2C20+2C21+2C22=12+22+12 = 6
Now check the options
(a) Does not hold given condition,
(b) (i) Put n = 1, then 2!1!1!=2
(ii) Put n = 2, then 4!2!2!=4×3×2×12×1×2×1=6
Note: Students should remember this question as identity
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