[tex]a_3=a_1+2r\\a_5=a_1+4r\\\\\left\{ {{a_1+2r=15}\atop{a_1+4r=19}}\right\\\left\{ {{-a_1-2r=-15}\atop{a_1+4r=19}}\right\ \ \ \ dodajemy\\\\2r=4\ \ czyli\ \ r=2\\\\a_1+2r=15\\a_1+4=15\\a_1=11\\\\S_n = \frac{2a_1+(n-1)*r}{2}*n\\S_n = \frac{22+(n-1)*2}{2}*n\\S_{20} = \frac{22+(19)*2}{2}*20\\S_{20} = 60*10=600[/tex]
