(k-1)x²+(k+4)x +k+7=0
a=k-1,b=k+4, c=k+7
D=b²-4ac, x1=x2⇔D=0
(k+4)²-4.(k-1)(k+7)=0
k²+8k+16-4(k²+6k-7)=0
k²+8k+16-4k²-24k+28=0
-3k²-16k+44=0
3k²+16k-44=0,D=16²-4.3.(-44)=256+528=784,√D=√784=28
k1=(-16+28)/6=12/6=2
k2=(-16-28)/6=-44/6=-22/3=-7 1/3
Используя основные свойства логарифмов:
![log_25+log_2\frac{1}{20}=log_2(5*\frac{1}{20})=log_2(\frac{1}{4})=log_2(2^{-2})=-2](https://tex.z-dn.net/?f=log_25%2Blog_2%5Cfrac%7B1%7D%7B20%7D%3Dlog_2%285%2A%5Cfrac%7B1%7D%7B20%7D%29%3Dlog_2%28%5Cfrac%7B1%7D%7B4%7D%29%3Dlog_2%282%5E%7B-2%7D%29%3D-2)