20. If the coefficient of 4th term in the expansion of   x   +   α 2 x n   i s   20 .   t h e n   t h e   r e s p e c t i v e   v a l u e s   o f   α   a n d   n   a r e  
a) 2,7              b) 5,8                   c) 3,6                                d) 2,6 20 ?

General term of the given expansionTr+1=Crn xn-r α2xrT4=C3n xn-3 α2x3Given: T4=C3n xn-3 α2x3=20C3n xn-6 α38=20 x0Comparing both sidesPower of x on both sides must be same n-6=0n=6Put n=6C36 x6-6 α38=20 x06×5×43×2×1 α38=20α38=1α3=8α=2
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