Question

without actual addiction find the sum 1+3+5+7+...+21 = ?​

Answer 1

Answer:

Step-by-step explanation:

Sn=n/2(a+(n-1)d)                               a=first term

                                                         n=no of terms             d=commondefference

S21=1/2(1+(21-1)1)

      =21/2(1+21)

       2*21/2=21