I bought a comic book, a fairy tale book. The story book is 29 yuan in total. The comic book and the fairy tale book are 17 yuan in total. The story book is 4 yuan more than the fairy tale book. What are the prices of the fairy tale book, the story book and the comic book

I bought a comic book, a fairy tale book. The story book is 29 yuan in total. The comic book and the fairy tale book are 17 yuan in total. The story book is 4 yuan more than the fairy tale book. What are the prices of the fairy tale book, the story book and the comic book

Fairy tale book 8 yuan, story book 12 yuan, comic book 9 yuan

There are 49 fairy tale books in the school library. Class 31 borrowed two seventh of them. How many books did class 31 borrow

Two out of seven of 49

There is a batch of cement on the construction site, 40% of which was transported in the first time and 35% in the second time. There are still 17 tons left. How much is this batch of cement?

Cement has
17÷（1-40%-30%）
=17÷30%
=170 / 3 tons
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How to prove that ax ^ 2 + BX + C = 0 has two roots (one positive and one negative) if and only if AC

According to Weida's theorem
x1+x2=-b/a
x1*x2=c/a
Because ax ^ 2 + BX + C = 0 has two roots (one positive and one negative)
So C / a < 0
That is to say, a and C must have opposite signs. Therefore, the necessary condition for the original proposition to be true is AC

The mass of pile a is 60% of that of pile B. if 4 tons of yellow sand are taken from each pile
At this time, the mass of pile a is 40% of that of pile B. how many tons of yellow sand are there in pile a and pile B?

Let B x be a 0.6x
0.4(x-4)=0.6x-4
x=16
B 12 tons, a 12 * 0.6 = 7.2 tons

It is known that f (x) is an odd function defined on the interval [- 1,1], and f (1) = 1. If m, n Euro [- 1,1], M + n is not equal to zero, then [f (m) + F (n)] / (M + n) > 0, the inequality f (x + 1 / 2) is solved

Odd function
f(1-x)=-f(x-1)
So f (x + 1 / 2)

There are 100 male workers in Mingming garment factory, and the number of female workers accounts for 3 / 5 of the total number of workers. How many workers are there in this garment factory?

The number of workers is X
Male workers account for 2 / 5,
2/5X=100
X=250
There are 250 workers

If the real number XY satisfies (x + y) (x + y + 3) + 2 = 0, then the value of X + y is?

(x+y)(x+y+3)+2=0
(x+y)²+3(x+y)+2=0
(x+y+1)(x+y+2)=0
x+y=-1
x+y=-2
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London will hold the Olympic Games in 2012

London will host the Olympus games on 2012

If x (x-1) - (x2-y) = 7, find the value of half x2 + Y2 XY

x(x-1)-(x2-y)=7
x²-x-x²+y=7
y-x=7
Half X & # 178; + Y & # 178; - XY
=Half (X & # 178; + Y & # 178; - 2XY)
=Half (X-Y) & 178;
=Half (- 7) & 178;
=49 / 2