Recall the formula
f ( a ) + f ′ ( a ) ( x − a ) + f ″ ( a ) 2 ( x − a ) 2 {\displaystyle f(a)+f'(a)(x-a)+{\frac {f''(a)}{2}}(x-a)^{2}}
for the second-degree Taylor polynomial for f ( x ) {\displaystyle f(x)} at x = a {\displaystyle x=a} .
