Explain the RHS and CPCT criterion of congruency.

Answer :

RHS (Right-angle Hypotenuse Side) congruency criterion is that, If two right triangles are such that the hypotenuse and one side of a triangle are equal to the hypotenuse and one side of the other triangle, then the two triangles are congruent.

As we have two triangles ABC and PQR 

And here 
$\angle$ ABC  =  $\angle$ PQR  = 90$°$
And
BC  =  QR  ,  AC  =  PR 
SO,
$∆$ ABC $\cong$ $∆$ PQR                                     ( by RHS rule ) 

CPCT ( corresponding parts of congruent triangles ) : It means that if two triangles are congruent with any of SAS, ASA, SSS and RHS congruence rules then remaining angles and sides corresponding in both triangles are equal.

As here we proved 

$∆$ ABC $\cong$ $∆$ PQR                                     ( by RHS rule ) 
Then
By CPCT 

AB  =  PQ  
$\angle$ ACB  =  $\angle$ PRQ  
And
$\angle$ BAC  =  $\angle$ QPR