∴AD=AQ,∠QAD=90

Q is QE⊥PD, and PD is E.

PQC⊥ aircraft dcq；;

∴E is the midpoint of PD == >; QD=QP,QD⊥QP

CD⊥ Yi Zhi surface AQPD== >CD⊥PQ.

∴PQ⊥ surface dry quenching

PQC dry quenching

(2) Analysis: Let the side length of ABCD be 1.

It is easy to know the PCD of BC⊥ plane = = > BC ⊥ PC.

∴bc=cd= 1,pd=2==>； PC =√5 = = & gt； PB=√6

Go through c, CF⊥PB from PB to f, q, QG⊥PB from PB to g, f, HF//QG from QB to h, and connect HC.

∴∠CFH is the plane angle of dihedral angle Q-BP-C.

bc^2=bf*bp==>； 1 = BF *√6 = = & gt； BF=√6/6== >CF=√(BC^2-BF^2)=√30/6

Yi Zhi BQ=DQ=PQ=√2

∴G is the midpoint of PB

QG=√(BQ^2-BG^2)=√2/2

⊿bfh∽⊿bgq==>； Blast furnace/blast furnace = blast furnace /QG = blast furnace /BQ

∴HF=√2/6,BH=√2/3

∵BC⊥BQ

∴ch=√(bc^2+bh^2)=√ 1 1/3

According to the cosine theorem HC 2 = FC 2+FH 2-2 * FC * HF * COS ∠ CFH.

1 1/9 = 5/6+ 1/ 18-2 *√30/6 *√2/6 * cos∠CFH

cos∠CFH=-√ 15/5

∴ The cosine Q-BP-C of dihedral angle is -√ 15/5.