Question

N= 13+2x(1+2+3...+12). 

Stiu ca era o formula dar nu-mi-o aduc aminte... 

Answer 1

N=13+2* (1+12)*12/2=

N=13+12*(1+12)

N=13+12*13

N=13(1+12)

N=13*13

N=13²=169

Answer 2

N=13+2x(1+2+3+....12)=13+2x* 12*(12+1):2=13+156 x
Formule este Suma lui Gauss
S=1+2+3+4+.......+n= n(n+1):2
Daca vrei sa o tii minte invata cum se deduce
Fie S=1 +2 + 3+ 4 + 5 scriem suma inversand ordinea termenilor
     S=5 +4 + 3 + 2 + 1
Adunam termen cu termen si obtin 2S= (1+5)+(2+4)+(3+3)+(4+2)+(5+1)= 6+6+6+6+6=5*6
⇒ S=5*6:2 adica daca avem n termeni S=n(n+1):2