40 nha bạn

đổi 75%=3/4.Ta có:

3/4xX+3/4xX+X=30

(3/4+3/4+1)xX=30

    5/2     x    X=30

                 X  = 30:5/2

                X   = 12

\(75\%\times x+\frac{3}{4}\times x+x=30\)

\(\frac{3}{4}\times x+\frac{3}{4}\times x+1\times x=30\)

\(\left(\frac{3}{4}+\frac{3}{4}+1\right)\times x=30\)

\(\frac{5}{2}\times x=30\)

\(x=30\div\frac{5}{2}\)

\(x=12\)

giá xăng từ 20 000 đồng lên 21 000 đồng 1 lít . hỏi giá xăng tăng bao nhiêu phần trăm? 

a,100%X-40%X=3,6

=>60%X=3,6

=>X=3,6:60%

=>X=6

b,30%X+100%=52

=>130%X=52

=>X=52:130%

=>X=40

ta có:\(\frac{1}{1.2}+\frac{1}{3.4}+...+\frac{1}{99.100}=1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\)

\(=\left(1+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{99}\right)-\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{100}\right)\)

\(=\left(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{100}\right)-2\left(\frac{1}{2}+\frac{1}{4}+...+\frac{1}{100}\right)\)

\(=\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{100}\right)-\left(1+\frac{1}{2}+\frac{1}{3}+...+\frac{1}{50}\right)=\frac{1}{51}+...+\frac{1}{100}\)

\(\frac{2012}{51}+\frac{2012}{52}+...+\frac{2012}{100}=2012\left(\frac{1}{51}+\frac{1}{52}+...+\frac{1}{100}\right)\)

bài toán được viết lại như sau:

\(\left(\frac{1}{51}+\frac{1}{52}+...+\frac{1}{100}\right).x=2012\left(\frac{1}{51}+\frac{1}{52}+...+\frac{1}{100}\right)\)

\(\Rightarrow x=2012\left(\frac{1}{51}+...+\frac{1}{100}\right):\left(\frac{1}{51}+...+\frac{1}{100}\right)\)

\(\Rightarrow x=2012\)

vậy x=2012

7) 30% . X + X = 52

<=>1,3X=52

=> x=52/1,3=40

Ta có  X + X = 52

          X x 2 = 52

          X       = 52 : 2

          X       = 26

xx +x+52=100

xx + x = 52 - 100

xx +x = 48

x *11 + x = 48

x *(11+1)=48

x * 12 = 48 

        x = 48 : 12 

           x= 4

3 x = 100 - 52 = 48

x = 48 : 3 = 16

Đáp số: x = 16

47 + 1 + 52 = 100

40 chắc chắn 100% luôn

Nhớ nhé

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40 la dung do

a) \(75\%\times x+\frac{3}{4}\times x+x=30\)

\(\frac{3}{4}\times x+\frac{3}{4}\times x+x=30\)

\(x\times\left(\frac{3}{4}+\frac{3}{4}+1\right)=30\)

\(x\times\frac{5}{2}=30\)

\(x=30\div\frac{5}{2}\)

\(x=30\times\frac{2}{5}\)

\(x=12\)

b) \(x-40\%=36\)

\(x-\frac{2}{5}=36\)

\(x=36+\frac{2}{5}\)

\(x=36\frac{2}{5}\)

c) \(30\%\times x+x=52\)

\(\frac{3}{10}\times x+x=52\)

\(x\times\left(\frac{3}{10}+1\right)=52\)

\(x\times\frac{13}{10}=52\)

\(x=52\div\frac{13}{10}\)

\(x=40\)