-21(4-10а)-54а=-84+210а=210а-84
A^2+b^2=(a+b)^2-2ab=49-24=25
2х+у=6
х-2у=-4 ||×(-2)
2х+у=6
-2х+4у=8
сложим
2х-2х+у+4у=6+8
5у=14
у=14/5=2 4/5=2.8
х=2у-4=2×2.8-4=1.6
(1.6;2.8)
(a² -25) /(2a² +10a) =(a -5)(a +5) /2a(a +5) =(a -5) /2a,
при а =0
(5х-1) * (5х+1) - 25х2 = 25X2-5X-5X-1-25X2 = -10X-1=0