Q21

Consider the given function $f(x)$.

$$f(x)=\left\{\begin{array}{ll} a x+b & \text { for } x<1 \\ x^{3}+x^{2}+1 & \text { for } x \geq 1 \end{array}\right.$$

If the function is differentiable everywhere, the value of $b$ must be ________. (rounded off to one decimal place)