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Calculati S=5+10+15+20+..........+400

Răspuns :

S=5(1+2+3+....80)=5*[(1+80)*80/2]=5*81*40=200*81= 16200
aflam ratia adica din cat in cat care este 5 r=5 n =(tn-t1 ):r+1 adica n=(400-5):5+1,n=395:5+1=80 si s= tn+t1*n:2 deci s=(400+5)*80:2,s=405*80:2=16200