Question

calculati suma primilor 100 termeni din ecuatia 7+11+15+19+......

Answer 1

7 + 11 + 15 + 19 + ........

( n - 7 ) : 4 + 1 = 100 ( termeni )

( n - 7 ) : 4 = 100 - 1

n - 7 = 99 × 4

n = 396 + 7

n = 403

S = 7 + 11 + 15 + 19 + ...... + 403

S = 100 × ( 7 + 403 ) / 2

S = 50 × 410

S = 20 500

Answer 2

7+11+15+19+.....+a₁₀₀

a₂ = 7+4 =11

a₃ = 7+2·4 =15

a₄ = 7+3·4 = 19

a₁₀₀ = 7 +99·4 = 403

S₁₀₀ = (a₁+a₁₀₀)· 100:2 = (a₁+a₁₀₀)·50 = (7 + 403)·50 = 410·50 = 20500