Ob=20
a=20:4=5
x=\frac{1}{2}d_1
y=\frac{1}{2}d_2
x^2+y^2=a^2
x^2+y^2=5^2
x^2+y^2=25
d_1+d_2=14
\frac{1}{2}d_1+\frac{1}{2}d_2=x+y=7
…
x^2+y^2=25
x+y=7
…
x^2+y^2=25
x=7-y
…
(7-y)^2+y^2=25
x=7-y
…
7^2-2*7*y+y^2+y^2=25
x=7-y
…
49-14y+2y^2=25
x=7-y
…
2y^2-14y+24=0
x=7-y
…
2y^2-14y+24=0
\Delta=b^2-4ac=196-4*2*24=196-192=4
\sqrt{\Delta}=2
y_1=\frac{-b-\sqrt{\Delta}}{2a}=\frac{14-4}{4}=2,5
y_2=\frac{-b+\sqrt{\Delta}}{2a}=\frac{14+4}{4}=4,5
x_1=7-2,5=4,5
x_2=7-4,5=2,5
d_1=2*4,5=9
d_2=2*2,5=5
$P=\frac{9*5}{2}=22,5$$cm^2$