Question

Calculati suma -4+3+10+17+.....+66​

Answer 1

Răspuns:

Explicație pas cu pas:

progresie aritmetica cu a1 = -4 si r = 7

66 = -4 + (n - 1)*7 = -4 + 7n - 7 = 7n - 11

66 + 11 = 7n

77 = 7n

n = 77 : 7 = 11

Sn = n*(a1 + an)/2

S11 = 11*(-4 + 66)/2 = 11*62/2 = 11*31 = 341

Answer 2

$\it -4=-4+7\cdot0\\ \\ 3\ =\ -4+7\cdot1\\ \\ 10=-4+7\cdot2\\ \\ 17=-4+7\cdot3\\ \\ \vdots \\ \\66=-4+7\cdot10\\ \rule{110}{0.7}\\ \\ S=-4\cdot11+7(1+2+3+\ ...\ +10)=-44+7\cdot55==-44+385=341$