ABCD is a rectangle and M is the midpoint of CD.The inradii of triangles ADM and ABM are 3 and 4 respectively.Then find the area of the rectangle. ABCD is a rectangle and M is the midpoint of CD.The inradii of triangles ADM and ABM are 3 and 4 respectively.Then find the area of the rectangle.
Let $AD=a,DM=b$ so $AM=\sqrt{a^2+b^2}$
In a triangle ,inradius$=r=\frac{abc}{4Rs}$
where $a,b,c$ are the side lengths and R,r,s are circumradius,inradius and semiperimeter respectively.
Using this relation in the triangle $ADM$,$r=\frac{ab\sqrt{a^2+b^2}}{4(\frac{a+b+\sqrt{a^2+b^2}}{2})\frac{\sqrt{a^2+b^2}}{2}}$
So,$3a+3b+3\sqrt{a^2+b^2}=ab$
Squaring both sides and after simplification we get
$6(a+b)=18+ab$
And the area of the rectangle is $2ab$.I need to find $2ab,$I am not able to find it.Please help me.
continuing from your method: $AB=2b$ and $AM=MB=\sqrt{a^2+b^2}$

Now $$Ar(\Delta ABM)=ab=r_{ABM}\times s_{ABM}=4\frac{(2b+2\sqrt{a^2+b^2})}{2}$$ So

$$ab=4(b+\sqrt{a^2+b^2}) \tag{1}$$ and from your equation we have

$$3a+3(b+\sqrt{a^2+b^2})=ab \tag{2}$$ $\implies$

$$3a+\frac{3ab}{4}=ab \implies 3+\frac{3b}{4}=b \implies b=12$$
In the rectangle, if $R$ is the point on $AB$ opposite to $M_y$, then $AR=x$.

If P be the point of contact of AM with bigger circle, then $AP=x$, and $Q$ is point of contact of $AD$ with smaller then $AQ=x$, as pair of tangents from A
So in triangle BMC, $PM=x+3$, $BR=x$ and $BM=2x-3$.

From the Pythagorean equation, 
$X=9$, $AB=18$, and $AD=12$.