c, Ta co tam giac AID = DKC (c.g.c) $\Rightarrow \widehat{AID}=\widehat{DKC}$Ma $\Rightarrow \widehat{AID}+\widehat{ADI}=90^0\Rightarrow \widehat{DKC}+\widehat{ADI}=90^0\Rightarrow $ CK ⊥ ID (1)
Lai co SI ⊥ (ABCD) nen SI ⊥ CK (2)
Tu (1) va (2) $\Rightarrow $ CK ⊥ (SID). Ma CK thuoc SCK nen (SCK) ⊥ (SID) (dpcm)