[tex]\displaystyle\bf\\\Big(5a+\frac{1}{15}b \Big)^{2} =\Big(5a\Big)^{2} +2\cdot 5a\cdot \frac{1}{15} b+\Big(\frac{1}{15} b\Big)^{2} =\\\\\\=25a^{2} +\frac{2}{3} ab+\frac{1}{225} b^{2}[/tex]