\ \ f(15)=\sqrt{15} \ "="4-\frac{1}{8}" alt="f(x)=\sqrt{x} \\
f(16)=4 \\
f(16-1)=f(16)-f'(16)\cdot1=\sqrt{15}\\
f(16)=4, \ \ f'(16)=\frac{1}{2\sqrt{16}}=\frac{1}{8} \\
f(16-1)=4-\frac{1}{8} \ \ => \ \ f(15)=\sqrt{15} \ "="4-\frac{1}{8}" align="absmiddle" class="latex-formula">