a21 = 30
r = 0,7
a21 = a1+20r
30 = a1+20*0,7
30 = a1+14
a1 = 30-14
a1 = 16
Sn = (a1+an)/2 * n
S21 = (16+30)/2 *21
S21 = 46/2 *21
S21 = 23*21
S21 = 483
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a21 = 30
r = 0,7
a21 = a1+20r
30 = a1+20*0,7
30 = a1+14
a1 = 30-14
a1 = 16
Sn = (a1+an)/2 * n
S21 = (16+30)/2 *21
S21 = 46/2 *21
S21 = 23*21
S21 = 483