Question

Se cunosc a1=5 si r=2.calculati suma primilor 5 termeni ai progresiei aritmetice ....????

Answer 1

a1=5
r=2
S5=?

a5=a1+4r
a5=5+4×2
a5=5+8
a5=13
S5=[(a1+an)n]/2
S5=[(5+13)5]/2
S5=(18×5)/2
S5= 90/2
S5=45