If the coefficient of x7 in ax2+1bx11 equals the coefficient of x-7 in ax-1bx211, then a and b satisfy the relation

General term in ax2+1bx11 is Tr+1=Cr   11ax211-r1bxr                                                                    =Cr   11a11-r1brx22-3r ∴To get coefficinet of x7, we must have 22-3r=7 ⇒r=5 ∴coefficient of x7=C5   11a61b5 Also,general term in ax-1bx211 is Tr+1=Cr   11-1rax11-r1bx2r                                                                             =Cr   11-1ra11-r1brx11-3r ∴To get coefficinet of x-7, we must have 11-3r=-7 ⇒r=6 ∴coefficient of x-7=C6   11a51b6 Given C5   11a61b5=C6   11a51b6 ⇒ab=1