Explicație pas cu pas:
(a+5)(b-2)=420 =>
ab+5b-2a-10=420 =>
420+5b-2a-10=420 =>
5b-2a-10=0 =>
5b-2a=10 => 2a=5b-10
a*b=420 => 2*a*b=840 =>(5b-10)b=840 =>
5b²-10b-840 =0 =>
5(b²-2b-168)=0 =>
b²-2b-168 =0 =>
b²-14b+12b-168=0 =>
b(b-14)+12(b-14)=0 =>
(b-14)(b+12)=0 <=>
b-14=0 =>b=14
sau
b+12=0 =>b=-12
=> 2a=5*14-10 => 2a=70-10 => a=30
sau
2a=5*(-12)-10 => 2a=-60-10 => 2a=-70 =>
a=-35
(a,b)€{(30,14);(-35,-12)}