Question

dau coronitaaaa! va rogggg

Answer 1

FORMULA:  n(n+1)/2  =>asta doar atunci cand ai 1+2+3+...+n

a)5+10+15+...+45=5(1+2+3+...+9)=5*9(9+1)/2=45*10/2=450/2=225

b)7+14+21+...+63=7(1+2+3+...+9)=7*9(9+1)/2=63*10/2=630/2=315

c)8+16+24+...+72=8(1+2+3+...+9)=8*9(9+1)/2=72*10/2=720/2=360

d)11+22+33+...+99=11(1+2+3+...+9)=11*9(9+1)/2=99*10/2=990/2=495