Q.
If the 4th term in the expansion of 2+3x810 has the maximum numerical value, then the range of values of x for which this will be true is
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a
−6421
b
2164
c
−6421
d
−2164
answer is A.
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Detailed Solution
T4 is numerically greatest term ⇒T4 T3>1,⇒T4 T5>1⇒T4 T3>1,T5 T4<1Now T4 T3=C3 10273x83C2 10283x82=x2,T5 T4=C4 10263x84C3 10273x83=21x64∴T4 T3>1⇒x2>1⇒|x|>2⇒x∈-∞,-2∪2,∞ →1and T5 T4<1⇒21x64<1⇒|x|<6421⇒x∈-6421,6421 →2By 1 and 2 we have x∈−6421,−2∪2,6421
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