Question

aratati ca nr. c 2^2+2^4+2^6....2^168 se divide prin 21

Answer 1

[tex]c=2^2+2^4+2^6+...+2^{168}\\ c=2^2(1+2^2+2^4)+2^8(1+2^2+2^4)+....+2^{164}(1+2^2+2^4)\\ c=(1+2^2+2^4)(2^2+2^8+....+2^{164})\\ c=21(2^2+2^8+...+2^{168})[/tex]