Given expansion is (31x31+2x321)18.
⇒T7=C618(31x31)12(2x321)6
⇒m=C618(31)12(21)6
⇒T13=C1218(31x31)6(2x321)12
⇒n=C1218(31)6(21)12
⇒nm=C1218(31)6(21)12C618(31)12(21)6
⇒nm=(21)6(31)6
⇒nm=(32)6
⇒(nm)31=94
⇒(mn)31=49
Let mandn be the coefficients of seventh and thirteenth terms respectively in the expansion of (31x31+2x321)18. Then (mn)31 is:
Held on 1 Feb 2024 · Verified 6 Jul 2026.
94
91
41
49
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