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[tex]\displaystyle\bf\\15)\\a)\\\\3^{15}+3^{16}+3^{17}=\\\\=3^{15}+3^{15+1}+3^{15+2}=\\\\=3^{15}+3^{15}\times3^1+3^{15}\times3^2=\\\\=3^{15}(1+3^1+3^2)=3^{15}(1+3+9)=3^{15}\times13~\vdots~13\\\\\\b)\\\\2^{22}+2^{24}+2^{26}=\\\\=2^{22}+2^{22+2}+2^{22+4}=\\\\=2^{22}+2^{22}\times2^2+2^{22}\times2^4=\\\\=2^{22}(1+2^2+2^4)=\\\\=2^{22}(1+4+16)=2^{22}\times21~\vdots~21[/tex]

Explicație pas cu pas:

A=3^15+3^16+3^17 Sa se imparta la 13.

A=(3^15+16+17) :13

A=(3^48)  :13

Amplificam cu 3 si va fi

A=48 care nu se imparte la 13