Cho hàm số f(x) xác định với mọi x khác 0 thỏa mãn
a, f(1)=1
b, f(\(\dfrac{1}{x}\))=\(\dfrac{1}{x^2}\).f(x)
c, f(x1+x2)=f(x1)+f(x2) với mọi x1,x2khác 0 và x1+x2 khác 0
C tỏ rằng f(\(\dfrac{5}{7}\))=\(\dfrac{5}{7}\)
K
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17 tháng 2 2020
Theo c) \(f\left(\frac{5}{7}\right)=f\left(\frac{2}{7}+\frac{3}{7}\right)=f\left(\frac{2}{7}\right)+f\left(\frac{3}{7}\right)\)
\(f\left(\frac{2}{7}\right)=f\left(\frac{1}{7}+\frac{1}{7}\right)=f\left(\frac{1}{7}\right)+f\left(\frac{1}{7}\right)=2.f\left(\frac{1}{7}\right)\)
\(f\left(\frac{3}{7}\right)=f\left(\frac{1}{7}+\frac{2}{7}\right)=f\left(\frac{1}{7}\right)+f\left(\frac{2}{7}\right)=f\left(\frac{1}{7}\right)+2f\left(\frac{1}{7}\right)=3.f\left(\frac{1}{7}\right)\)
\(\implies\)\(f\left(\frac{5}{7}\right)=5.f\left(\frac{1}{7}\right)\) (1)
Theo b) \(f\left(\frac{1}{7}\right)=\frac{1}{7^2}.f\left(7\right)\) (2)
Theo c) \(f\left(7\right)=f\left(3+4\right)=f\left(3\right)+f\left(4\right)\)
\(=2.f\left(3\right)+f\left(1\right)\)
\(=6.f\left(1\right)+f\left(1\right)\)
\(=7.f\left(1\right)\)
Theo a)\(f\left(1\right)=1\)\(\implies\)\(f\left(7\right)=7\) (3)
Từ (1);(2);(3)
\(\implies\) \(f\left(\frac{5}{7}\right)=\frac{5}{7}\)
KN
4 tháng 3 2020
Từ giả thiết \(f\left(x_1+x_2\right)=f\left(x_1+x_2\right)\) ta có các biến đổi sau:
\(f\left(2020\right)=f\left(1024\right)+f\left(996\right)\)
\(=f\left(1024\right)+f\left(512\right)+f\left(484\right)\)
\(=f\left(1024\right)+f\left(512\right)+f\left(256\right)+f\left(228\right)\)
\(=f\left(1024\right)+f\left(512\right)+f\left(256\right)+f\left(128\right)+f\left(100\right)\)
\(=f\left(1024\right)+f\left(512\right)+f\left(256\right)+f\left(128\right)+f\left(64\right)\)
\(+f\left(36\right)\)
\(=f\left(1024\right)+f\left(512\right)+f\left(256\right)+f\left(128\right)+f\left(64\right)\)
\(+f\left(32\right)+f\left(4\right)\)
Dễ tính \(f\left(1024\right)=\)\(2.f\left(512\right)=4.f\left(256\right)=8.f\left(128\right)=16.f\left(64\right)\)
\(=32.f\left(32\right)=64.f\left(16\right)=128.f\left(8\right)=256.f\left(4\right)=512.f\left(2\right)\)
\(=1024.f\left(1\right)=1024\)
Tương tự ta có \(f\left(512\right)=512;f\left(256\right)=256;f\left(128\right)=128;f\left(64\right)=64;\)
\(f\left(32\right)=32;f\left(4\right)=4\)
\(\Rightarrow f\left(1024\right)+f\left(512\right)+f\left(256\right)+f\left(128\right)+f\left(64\right)\)
\(+f\left(32\right)+f\left(4\right)=2020\)
hay \(f\left(2020\right)=2020\)
Ta có: \(f\left(\frac{1}{x}\right)=\frac{1}{x^2}.f\left(x\right)\)
\(\Rightarrow f\left(\frac{1}{2020}\right)=\frac{1}{2020^2}.2020=\frac{1}{2020}\)
\(\Rightarrow f\left(\frac{3}{2020}\right)=f\left(\frac{2}{2020}\right)+f\left(\frac{1}{2020}\right)\)
\(=f\left(\frac{1}{2020}\right)+f\left(\frac{1}{2020}\right)+f\left(\frac{1}{2020}\right)\)
\(=\frac{1}{2020}.3=\frac{3}{2020}\)
Vậy \(f\left(\frac{3}{2020}\right)=\frac{3}{2020}\)
Phần này khó chú ý nè bạn
Giải
Ta có f(x1+x2) = f(x1) + f(x2)
nên f(7) = f(3)+f(4)= f(2)+f(1) + f(2)+f(2) = f(1)+f(1)+f(1)+f(1)+f(1)+f(1)+f(1)=7
\(f\left(\dfrac{1}{7}\right)=\dfrac{1}{49}.f\left(7\right)=\dfrac{1}{49}.7=\dfrac{1}{7}\)
Ta có :\(f\left(\dfrac{5}{7}\right)=f\left(\dfrac{2}{7}\right)+f\left(\dfrac{3}{7}\right)=f\left(\dfrac{1}{7}\right)+f\left(\dfrac{1}{7}\right)+f\left(\dfrac{1}{7}\right)+f\left(\dfrac{2}{7}\right)=f\left(\dfrac{1}{7}\right)+f\left(\dfrac{1}{7}\right)+f\left(\dfrac{1}{7}\right)+f\left(\dfrac{1}{7}\right)+f\left(\dfrac{1}{7}\right)=\dfrac{1}{7}+\dfrac{1}{7}+\dfrac{1}{7}+\dfrac{1}{7}+\dfrac{1}{7}=\dfrac{5}{7}\)
Đức cường sai rồi
ai cho f(1/7) =1/7 đâu?