Question 350075
(a)S1= 1 +2 +3+...+ 250  |sum with the series reversed, 250 terms equals to
_____+250+249+248+..+1    | 251
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=250*251=62750=2*S1 => S1=31375
 

(b) 5+10+15+...+305=5*(1+2+3+...61)=5*S2,
 
S2=__1+2+3+...61
+S2=61+60+.....1
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=62*61=2*S2
 
=>S2=31*61=1830+61=1891 => 5*S2=5000+4000+450+5=9455(=310*313/2)