\[\boxed{\mathbf{569.еуроки - ответы\ на\ пятёрку}}\]
\[S = p \cdot r;\ \ p - полупериметр;\ \]
\[\ r - радиус\ вписанной\ \]
\[окружности.\]
\[p = \frac{a + b + c}{2}.\]
\[\textbf{а)}\ P = 56;\ \ S = 84:\]
\[p = 56\ :28;\]
\[r = \frac{S}{p} = \frac{84}{28} = 3.\]
\[\textbf{б)}\ P = 144;\ \ r = 3,5:\]
\[p = 144\ :2 = 72;\]
\[S = 72 \cdot 3,5 = 216 + 36 = 252.\]
\[\textbf{в)}\ b = 15;c = 20;\ r = 2;\ \ \]
\[S = 42:\]
\[p = \frac{S}{r} = \frac{42}{2} = 21;\]
\[P = 21 \cdot 2 = 42;\]
\[a = 42 - (b + c) =\]
\[= 42 - (15 + 20) = 7.\]