Sn= (a1+an)/2 * n
a1=4-6=-2
a20= 4*20-6=74
S20= (-2+74)/2 * 20= 72*10=720
S20= a1+a20/2*20
a1 = 4-6=-2
a20= 80-6=74
s20= (-2+74)/2*20
i trzeba to tylko wyliczyc
s20= 72*10 = 720
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Sn= (a1+an)/2 * n
a1=4-6=-2
a20= 4*20-6=74
S20= (-2+74)/2 * 20= 72*10=720
S20= a1+a20/2*20
a1 = 4-6=-2
a20= 80-6=74
s20= (-2+74)/2*20
i trzeba to tylko wyliczyc
s20= 72*10 = 720