23. Oblicz sume
a. 1+2+...+90
b. 11+12+...+70
a)
a1=1
r= a2-a1-2-1=1
S90 = 91*90/2 = 4095
b)
a1=11
r=a2-a1= 12-11 = 1
an=70
S60 = (11+70)*60/2 = 2430
Ps. warto skorzystać z kalkulatora
http://kalkulator.sle.pl/ciag_arytmetyczny.html
II sposob
(1+89)+(2+88)+...+(44+46)+45+90=44*90+135=4095
(11+69)+(12+68)+...+(39+41)+40+70=29*80+110=2430
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a)
a1=1
r= a2-a1-2-1=1
S90 = 91*90/2 = 4095
b)
a1=11
r=a2-a1= 12-11 = 1
an=70
S60 = (11+70)*60/2 = 2430
Ps. warto skorzystać z kalkulatora
http://kalkulator.sle.pl/ciag_arytmetyczny.html
II sposob
(1+89)+(2+88)+...+(44+46)+45+90=44*90+135=4095
(11+69)+(12+68)+...+(39+41)+40+70=29*80+110=2430