In `triangleAOB and triangleCOD`,
`angle1=angle2` (alternate angles)
`angle3= angle4` (alternate angles)
`triangleAOB ~triangleCOD` ( AA corollary)
`(ar(triangleAOB))/(ar(triangleCOD))= (AB^(2))/(CD^(2))`
(ratio of area of two similar triangles is equal to the ratio of square of that corresponding sides)
`((2CD)^(2))/(CD^(2))` ( AB = 2CD, given )
`4/1`
`84/(ar(triangleCOD))=4/1 Rightarrowar(triangleCOD)= 84/4cm^(2)=21cm^(2)`