Spring sleep never dawns, you can hear birds singing everywhere. The sound of wind and rain comes at night, and you know how many flowers have fallen. an=1 4*n

A21=(5 85)*21/2

=45*21

=945 The average increment is the first term plus the last term multiplied by the number of terms divided by 2. The average is the sum divided by the number of terms, so the final term is the first term Add the last term multiplied by the number of terms divided by 2.