Способ сложения:
{ 4x + 7y = 90 | × 3
{-6x + 5y = 20 | × 2
{ 12x + 21y = 270
{-12x + 10y = 40
12х + 21у + ( - 12х + 10у) = 270 + 40
31у = 310
у = 310÷31
у = 10
4х + 7×10 = 90
4х + 70 = 90
4х = 90 - 70
4х = 20
х = 20 ÷ 4
х = 5
Способ подстановки :
{4x + 7y = 90
{-6x + 5y = 20 ⇒ 5y = 20 +6x ⇒ y = 2(10+3x)/5 ⇒ y =0,4(10+3x)
4x + 7× 0,4(10+3x) = 90
4x + 2,8(10+3x) = 90
4x + 28 + 8,4x = 90
12,4x = 90 - 28
12,4x = 62
х = 62 ÷ 12,4
х = 5
y = 0,4×(10 + 3×5) = 0,4 × 25
у = 10
ответ : ( 5 ; 10) .
1) (х-1)² = х²-2х*1+1² = х²-2х+1
2) (-х-1)² = (-х)²+2х+1 = х²+2х+1
4(x+2)=7(2x-1)-9(3x-4)+30
4х+8=14х-7-27х +36+30
4х-14х+27х=-7+36+30-8
17х=51
х=51/17
х=3.
2) 3(2y+1)=5(12y-7)-7(6y-1)+23
6у+3=60у-35-42у+7+23
6у-60у+42у=-35+7+23-3
-12у=-8
у=-8/(-12)
у=2/3.
3) 0,3(0,4y-1,2)=-0,36y+3,096
0,12у-0,36=-0,36у+3,096
0,12у+3,6у=з,096+0,36
3,72у=3,456
у=3,456/3,72
у=0,288/0,31 .
4) 1,3(y-0,7)-0,12(y+10)=5y-9,75
1,3у-0,91-0,12у-1,2=5у-9,75
1,3у-0,12у-5у=0,91+1,2-9,75
-3,82у=-7,64
у=2