191. Perform the operations: a) $(5+3 i)(5-3 i) ;$ b) $(2+5 i)(2-$ $-5 i)$; c) $(1+i)(1-i)$.
Solution. a) $(5+3 i)(5-3 i)=5^{2}-(3 i)^{2}=25-9 i^{2}=25+9=34$;

b) $(2+5 i)(2-5 i)=2^{2}-(5 i)^{2}=4+25=29$;

c) $(1+i)(1-i)=1^{2}-i^{2}=1+1=2$.

192-197. Perform the operations;