Question

efectuati.
a. 7+9+11+...+2013-5-7-9-...-2011
c.5+10+15+...+95
b.11+12+13+...+78
d.5+8+11+...+305
vreau rezolvare !

Answer 1

a. 7+9+11+...+2013-5-7-9-...-2011 = 1006² -(1+3+5)=1012027
a1= 2*4-1=7
an= 2011 = 2*x-1 ; 2x= 2012; x=1006

c.5+10+15+...+95=5*(1+2+3+...18)=5*41*40/2= 4100

b.11+12+13+...+78 = (78-10)*(78+11)/2=3026

d.5+8+11+...+305=(305+5)*102/2=310*51=15810
a1=3*2-1=5
a2=3*3-1=8
an=3*x-1
3*x=305+1
3x=306
x=102