Question

[Nguyễn Huyền Trâm]

Cho \(\dfrac{a}{b}=\dfrac{c}{d} \) . Chứng minh :

a, \(\dfrac{a^2+c^2}{b^2+d^2} =\dfrac{ac}{bd}\)

b, \(\dfrac{a^2+c^2}{b^2+d^2} = \dfrac{a^2-c^2}{b^2-d^2}\)

c, \(\dfrac{(a+c)^2}{(b+d)^2} = \dfrac{(a-c)^2}{b-d)^2}\)

d, \(\dfrac{a^2+b^2}{a^2-b^2} = \dfrac{c^2+d^2}{c^2-d^2}\)

e, \(\dfrac{(a-b )^2}{(c-d)^2} = \dfrac{a^2+b^2}{c^2+d^2}\)

Answer 1

[Nguyễn Kim Hưng]

a)Áp dụng tính chất của dãy tỉ số bằng nhau, ta có:

\(\frac{a}{b}=\frac{c}{d}\) =>\(\frac{a}{c}=\frac{b}{d}\)

=>\(\frac{ac}{bd}=\frac{a^2}{c^2}=\frac{b^2}{d^2}=\frac{a^2+b^2}{c^2+d^2}\)

=>\(\frac{ac}{bd}=\frac{a^2+b^2}{c^2+d^2}\)

Answer 2

[Nguyễn Việt Lâm]

\(\frac{a}{b}=\frac{c}{d}\Rightarrow\frac{a^2}{b^2}=\frac{c^2}{d^2}=\frac{a^2+c^2}{b^2+d^2}=\frac{a^2-c^2}{b^2-d^2}\)

\(\frac{a}{b}=\frac{c}{d}=\frac{a+c}{b+d}=\frac{a-c}{b-d}\Rightarrow\frac{\left(a+c\right)^2}{\left(b+d\right)^2}=\frac{\left(a-c\right)^2}{\left(b-d\right)^2}\)

\(\frac{a}{b}=\frac{c}{d}\Rightarrow ad=bc\Rightarrow\frac{a}{c}=\frac{b}{d}\Rightarrow\frac{a^2}{c^2}=\frac{b^2}{d^2}=\frac{a^2+b^2}{c^2+d^2}=\frac{a^2-b^2}{c^2-d^2}\Rightarrow\frac{a^2+b^2}{a^2-b^2}=\frac{c^2+d^2}{c^2-d^2}\)

\(\frac{a}{c}=\frac{b}{d}=\frac{a-b}{c-d}\Rightarrow\frac{a^2}{c^2}=\frac{b^2}{d^2}=\frac{\left(a-b\right)^2}{\left(c-d\right)^2}=\frac{a^2+b^2}{c^2+d^2}\)