1)(5a +2) (5a-2) -5a(5a-1)=21
1)25a²-4-25a²+5a=21
5a=25
a=5
2)3(y+2)kv +(2y-1)kv -7(y+3) (y-3)=28
a)kv=0
-7(y+3) (y-3)=28
y²-9=-4
y²=5⇒y=-√5 y=√5
kv≠0
3kvy+6kv+2kvy-kv-7y²+63-28=0
7y²+5kvy+35+kv=0
D=25k²v²-27(35+kv)=25k²v²-27kv-945
1)25k²v²-27kv-945<0=нет решения<br>2)25k²v²-27kv-945=0
y=-5kv/14
3)25k²v²-27kv-945>0
y1=(-5kv-√(25k²v²-27kv-945))/14
y2=(-5kv+√(25k²v²-27kv-945))/14