∴69 1 10 12 1 122222 e 18。 ( 1)∵ PAABCD？ The plane ∴ Pabu? Polychlorinated diphenyl ethers? Aircraft ∴ PCBD? ∴ BDPAC？ Plane (2) Let the intersection of AC and BD be O and connect OE∶PC bde? Aircraft ∴ PCOE? BOPAC again? Plane ∴ PCBO? ∴ PCBOE？ Plane ∴ PCBE? Beo? Is the plane angle of dihedral BPCA BDPAC? Plane ∴ BDAC ∴ ABCD quadrilateral is a square ∴ 2BO? At PAC? ， 12332 oepaeocac？ ∴ Tambo Beo? The tangent of the plane angle of dihedral angle BPCA is 3 19. (1) Is it11221nnnsa? Zhong Ling 1n? DE: 2 1222 1Sa？ Do 2n? DE: 32322 1Sa？ Solution: 2 123aa, 3 16 13aa and 21325aa? The solution is 1 1a? (2) From11221nnnsa? 2 122 1 nnnsa 12 132nnnaa? And 12 1, 5aa also satisfies 12 132aa, so/kloc-0 132nnnaanN? Yes ∴11+232nnnaa ∴ 23nnna ∴ 32nnna (3) (method1) ∵12321/kloc-. kloc-0/ 1 1 1 1 13 ... 1 ...8+0322322 nnnnnnaa∴ 1 1 1 12 nnaa？ When 2n? ，32 1 1 12aa 43 1 12aa 54 1 12aa。