tolong yah !!pakai cara

pembuktian dgn cara deret aritmatika
a= 5
b = 4
Un = 4n+1

Sn = 1/2n (a+Un)
     = 1/2 n (5+4n+1)
     = 1/2n (4n+6)
     = n (2n+3)... sesuai

jika n = k
     Uk = 4k+1  --> Sk = k(2k+3)
    
      n = k+1 ---> S(k+1) = (k+1)(2(k+1)+3)
                                      = (k+1)(2k+2+3)
                                      = (k+1)(2k+5)
                                      = 2k²+5k+2k+5
                                      = 2k²+7k+5