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The mathematics of paper is like a rectangular piece of paper with a length of 20 cm. After two pieces of paper are partially overlapped.
∵Rt△BDE≌Rt△BCD≌Rt△ABD

∴∠FBD=∠CBD

∫ AD ∨ BC

∴∠CBD=∠FDB

∴∠FDB=∠FBD

∴BF=DF

At Rt△ABF.

AB？ +AF？ =BF？

15? +(20-BF)？ =BF？

BF= 15.625( 125/8)

AF = 20- 15.625 = 4.375(20- 125/8 = 35/8

∴S△BDF (overlapping) = 1/2S rectangle -S△ABF

= 1/2×20× 15- 1/2× 15×4.375

= 150-32.8 125

= 1 17. 1875

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