Consider the given rhombus ABCD
Let AC&BD be the diagonals of rhombus ABCD
In ΔAOD by phythagoras theorem,
AB2=AO2+DO2
By multiplying by 4 on both side and we get,
4AB2=4AO2+4DO2
We know that, the all sides of rhombus are equal
Then, AB=BC=CD=DA
So, AB2+BC2+CD2+DA2=4AO2+4DO2
Hence proved.