1. Expand

   $$(x^2+a+1)(x^2-a+1)$$

2. Let

   $$f(x)=\sqrt{x^2+1},x\in \mathbb{R}$$

   Solve the equation

   $$f(x)-100f^{″}(x)=0$$

Part (a) helps to factorise part (b).

Give the product of the real roots found in part (b).