\[\boxed{\text{403\ (403).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[\textbf{а)}\ x^{5} \cdot x^{8} = x^{5 + 8} = x^{13}\]
\[\textbf{б)}\ a^{6} \cdot a^{3} = a^{6 + 3} = a^{9}\ \]
\[\textbf{в)}\ y^{4} \cdot y^{9} = y^{4 + 9} = y^{13}\]
\[\textbf{г)}\ b^{8} \cdot b^{15} = b^{8 + 15} = b^{23}\]
\[\textbf{д)}\ x^{9} \cdot x = x^{9 + 1} = x^{10}\]
\[\textbf{е)}\ y \cdot y^{12} = y^{1 + 12} = y^{13}\]
\[\textbf{ж)}\ 2^{6} \cdot 2^{4} = 2^{6 + 4} = 2^{10}\]
\[\textbf{з)}\ 7^{5} \cdot 7 = 7^{5 + 1} = 7^{6}\]