a)
$\begin{array}{l}
a,b > 0\\
\dfrac{1}{a} + \dfrac{1}{b} = 1 = > a + b = ab\\
\left\{ \begin{array}{l}
a,b > 0\\
a + b = ab
\end{array} \right. = > a,b \ge 1\\
< = > a + b = a + b + 2\sqrt {(a - 1)(b - 1)} - 2\\
< = > 1 = \sqrt {(a - 1)(b - 1)} = \sqrt {(ab - (a + b) + 1)} = \sqrt {ab - ab + 1} = 1(dpcm)
\end{array}$
b)
$\begin{array}{l}
1)U;S\\
0.1 \to A;0.2 \to B;0,3 \to C;0.6 \to D\\
A = C - 9B + 4A:D = D + A:B = A - 9C + 4B:D = D + B:C = B - 9A + 4C:D = D + C\\
CALC = = = = = \\
2)U;P\\
0.1 \to A;0.2 \to B;0.3 \to C;(0.1*0.2*0.3) \to D\\
A = C - 9B + 4A:D = D*A:B = A - 9C + 4B:D = D*B:C = B - 9A + 4C:D = D*C
\end{array}$
