Relation between Coefficient and Indices of X and Y
If the coeffi...
Question
If the coefficients of second, third and fourth term in the expansion of (1+x)2n are in A.P, then 2n2−9n+7 is equal to
A
-1
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B
1
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C
3/2
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Solution
T2=2nC1x,T3=2nC2x2,T4=2nC3x3 Coefficient of T1,T2,T are in A.P. ⇒2.2nC2=2nC1+2nC3 ⇒22n!2!(2n−2)!=2n!(2n−2)!+2n!3!(2n−3)! ⇒2.2n(2n−1)2=2n+2n(2n−1)(2n−2)6 ⇒n(2n−1)=n+(n)(2n−1)(2n−2)6 ⇒6(2n2−n)=6n+4n3−6n2+2n ⇒6n(2n−1)=2n(2n2−3n+4) ⇒6n−3=2n2−3n+4 ⇒0=2n2−9n+7⇒2n2−9n+7=0