4 По Пифагору
AC² + BC² = AB²
x² + x² = 26²
x² = 26²/2
x = 26/√2 = 13√2
S = 1/2*x*x = 1/2*13²*2 = 13² = 169
7
D - центр вписанной окружности, и AD = BD = CD = 10
AB = 20
AC²+BC² = AB²
AC²+16² = 20²
AC² = 20²-16² = (20-16)*(20+16) = 4*36 = 144
AC = 12
S = 1/2*AC*BC = 1/2*12*16 = 96