Question

sa se calculeze sumele: a)Sn=1+3+5+7+...+(2n+1)
b)Sn=3+7+11+...+(4n-1)
c)Sn=2x+(2x-3)+(2x-6)+...+(2x-3n+3)

Answer 1

Aici ai de a face scu progresii aritmetice
$s = \frac{(a(0) + a(n)) \times r}{2}$
r-pasul progresiei

a)progresie aritmetică cu pasul 2
$s = \frac{(1 + 2n + 1)2}{2} = 2n + 2$


b)pasul 4
$s = \frac{(1 + 2n + 1)4}{2} = 4n + 4$
c)pasul -3

$s = \frac{(2x + 2x - 3n + 3) \times (- 3)}{2} = \frac{12x + 9n - 9}{2}$