We have, (1+ax+bx2)(1−2x)18=a0+a1x+a2x2+.......
=(1+ax+bx2)(1−18C1⋅2x+18C2⋅22x2+.....18C18(2x)18)
⇒a3=−18C3×8+18C2×4a−36b
⇒a4=18C4×16−18C3×8a+18C2×4×b
102a−6b=1088...(i)
64a−6b=480...(ii)
Solution of the above two equation gives.
a=16
b=3272