Reduce[{1/2 (x + y) a == 28, 1/2 (a + b) y == 35, 1/2 x b == 21, (x + y) (a + b) == 28 + 35 + 21 + 56, x > 0, y > 0, a > 0, b > 0}, {x, y, a, b}] // Simplify
$x>0\land x=y\land a=\frac{56}{x+y}\land b=\frac{84}{x+y}$
Bazi cozumler:
FindInstance[{1/2 (x + y) a == 28, 1/2 (a + b) y == 35, 1/2 x b == 21, (x + y) (a + b) == 28 + 35 + 21 + 56, x > 0, y > 0, a > 0, b > 0}, {x, y, a, b}, 4]
$\left\{\{x\to 14,y\to 14,a\to 2,b\to 3\},\{x\to 1,y\to 1,a\to 28,b\to 42\}\left\{x\to 89,y\to 89,a\to \frac{28}{89},b\to \frac{42}{89}\right\},\left\{x\to 134,y\to 134,a\to
\frac{14}{67},b\to \frac{21}{67}\right\},\right\}$