Question

determine a soma dos múltiplos de 9 existem entre 100 e 1000

Answer 1

900 ÷ 9 = 100

an = 999

a₁ = 108

r = 9

an = a₁ + (n - 1) r ---- 999 = 108 + (n - 1) . 9 ----999 - 108 = 9n - 9

891 = 9n - 9 ----- 9n = 891 + 9 ----- 9n = 900 ---- n = 900/9 ---- n = 100


Sn = [(a₁ + an) . n]/2  -------- S₁₀₀ = [(108 + 999) . 100]/2 --- s₁₀₀ = 1107 . 50

S₁₀₀ = 55 350