∴AB= 10√2

∫◇ABCD, e is the midpoint of AB, and f is the midpoint of BC.

∴AE=AH=BE=BF=5√2

∫∠ABC = 120, rectangular EFGH, rhombic ABCD

∴ae=ah=eh=5√2 ef = 1/2 * AC = 5√6

∴S rectangle EFGH=5√2*5√6=50√3

∫P = 10 yuan/㎡.

∴ Need to invest 50√3* 10=500√3 (yuan)