Let a1,a2,a3,....,a11 be real numbers satisfying a1=15,27−2a2>0 and ak=2(ak−1)−ak−2 for k=3,4,..,11 . If a21+a22+..=a21111=90, then the value of a−1+a2+..+a1111 is equal to
Open in App
Solution
Formula of ∑nr=1r=n(n+1)2 and ∑nr=1r2=n(n+1)(2n+1)6 are used.