Число увеличили на 7 21 / и получили 15 Найдите исходное число. Решение.

1)3х2-х+21=5х2
5х2-3х2+х-21=0
2х2+х-21=0
Д=1-4*(-21)*2=1+168=169=13 в кв
х1=-1-13\4=-14\4=-7\2=-3,5
х2=-1+13\4=12\4=3
2)х2+14х-25=-4х2+35х-47
х2+4х2+14х-35х-25+47=0
5х2-21х+22=0
Д=441-4*5*22=441-440=1
х1=21-1\10=20\10=2
х2=21+1\10=22\10=2,2
3)5х2+5х-15=2х2+11х+9
5х2-2х2+5х-11х-15-9=0
3х2-6х-24=0
х2-2х-8=0
Д=4-4*(-8)=4+32=36=6 в кв
х1=2-6\2=-4\2=-2
х2=2+6\2=8\2=4
4)4х2-19х+32=-6х2-6х+41
4х2+6х2-19х+6х+32-41=0
10х2-13х-9=0
Д=169-4*(-9)*10=169+360=529=23 в кв 
х1=13-23\20=-10\20=-1\2=-0,5
х213+23\20=36\20=1,8
5)3х2-10х+30=7х2+2х+3
7х2-3х2+2х+10х+3-30=0
4х2+12х-27=0
Д=144-4*(-27)*4=144+432=576=24 в кв
х1=-12-24\8=-36\8=-4,5
х2=-12+24\8=12\8=1,5
6)4х2-3х-12=3х2-6х-14
4х2-3х2-3х+6х-12+14=0
х2+3х+2=0
Д=9-4*2=1
х1=-3-1\2=-4\2=-2
х2=-3+1\2=-2\2=-1
7)-х2+3х+55=(х+7) в кв
-х2+3х+55=х2+14х+49
х2+х2+14х-3х+49-55=0
2х2+11х-6=0
Д=121-4*(-6)*2=121+48=169=13 в кв
х1=-11-13\4=-24\4=-6
х2=-11+13\4=2\4=1\2=0,5
8)-3х2-х+8=(х-3) в кв
-3х2-х+8=х2-6х+9
х2+3х2-6х+х+9-8=0
4х2-5х+1=0
Д=25-4*4*1=25-16=9=3 в кв
х1=5-3\8=2\8=1\4=0,25
х2=5+3\8=8\8=1
9)(х-1) в кв =2х2-6х-31
х2-2х+1=2х2-6х-31
2х2-х2-6х+2х-31-1=0
х2-4х-32=0
Д=16-4*(-32)*1=16+128=144=12 в кв
х1=4-12\2=-8\2=-4
х2=4+12\2=16\2=8
10)(х-9) в кв  = -х2+15х+50
х2-18х+81=-х2+15х+50
х2+х2-18х-15х+81-50=0
2х2-33х+31=0
Д=1089-4*31*2=1089-248=841 = 29 в кв
х1=33-29\4=4\4=1
х2=33+29\4=62\4=15,5
11)(х-7) в кв = 2х2+11х+23
х2-14х+49=2х2+11х+23
2х2-х2+11х+14х+23-49=0
х2+25х-26=0
Д=625-4*1*(-26)=625+104=729=27 в кв 
х1=-25-27\2=-52\2=-26
х2=-25+27\2=1 

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