If the coefficient of x7 in ax2-1bx11 is equal to the coefficient of x-7 in ax-1bx211, then ab=

Detail Solution:- For the coefficient of x7 in ax2-1bx11;r=22-73=153=5 - coefficient of x7 is -C5   11a61 b5 .  For the coefficient of x-7 in ax-1bx211;r=11-73=6 coefficient of x-7 is C6   11a51 b6 .  The coefficients are equal ⇒-C5   11a6 b5=C6   11a5 b6⇒-C6   11a=C6   111b⇒ab=-1