1. x^{2} - 9x + 20 = 0
x1, 2 = ( 9 +- \sqrt{81 - 80} ) / 2; x1 = 4; x2 = 5
2. x^{2} - 9x = 0 ; x = 9
3. x^{2} - 9x + 20 = 12
x^{2} - 9x + 6 = 0
x1, 2 = ( 9 +- \sqrt{81 - 24} ) / 2; x1 \approx 0.73; x2 \approx 8.27
4. x^{2} - 9x - 10 = 0
x1, 2 = ( 9 +- \sqrt{81 + 40} ) / 2; x1 = 10; x2 = 5
2<x<2,5
1<y<2
1)<span>2<x<2,5
-2<-y<-1
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0<x-y<1,5
2)</span>2<x<2,5
<span>1<y<2
---------------------------
2<xy<3,75
5</span>≤a≤6
5√2≤a√2≤6√2