Answer
(a) $f(\sqrt 5, 2, \pi, -3\pi) = 80\sqrt \pi$
(b) $f(1, 1,..., 1) = \Sigma^n_{k = 1}k=n(n+1)/2$
Work Step by Step
(a) $f(x, y, z, t) = x^2y^3\sqrt (z+t)$
$f(\sqrt 5, 2, \pi, -3\pi)$
Substituting $x = \sqrt 5, y = 2, z = \pi, t = 3\pi$ into the function
$f(\sqrt 5, 2, \pi, -3\pi) = (\sqrt 5)^2(2)^3\sqrt (\pi + 3\pi)$
$f(\sqrt 5, 2, \pi, -3\pi) = (5)(8)\sqrt (4\pi) = 40\times2\sqrt (\pi)$
$f(\sqrt 5, 2, \pi, -3\pi) = 80\sqrt \pi$
(b) $f(x_{1}, x_{2},..., x_{n}) = \Sigma^n_{k = 1}kx_{k}$
$f(1, 1,..., 1) = \Sigma^n_{k = 1}k(1)_{k}$
Substituting $x_{1} =1, x_{2} =1, x_{n} =1$ into the function
$f(1, 1,..., 1) = \Sigma^n_{k = 1}k = n(n+1)/2$
