Take DC midpoint h

Connect EH, FH,

E and h are the midpoint between AD and DC.

∴ EH//AC EH=(AC)/2= 1

F and h are the midpoint between BC and DC.

∴ HF//DB HF=(BD)/2= 1

∴ ∠EHF is the angle formed by AC and BD or its adjacent complementary angle.

At △EHF, the cosine theorem is used.

cos∠EHF =( 1+ 1-3)/(2 * 1 * 1)=- 1/2

∠EHF= 120

The angles formed by straight lines on different planes are acute or right angles.

The angle formed by AC and BD is 60.