#1&2)
a + (a+d) + (a+2d) + ... + (a+(n-2)d) + (a+(n-1)d) + (a+nd) = (n+1)(a + a + nd)/2
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#4&5)
S=c+cr+cr^2+....+cr^(n-1)
rS= cr+cr^2+....+cr^(n-1)+cr^n
S-rS=c-cr^n
S(1-r)=C(1-r^n)
S=(c(1-r)^n)/(1-r)
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