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Bài 2:
Đặt a/b=c/d=k
=>a=bk; c=dk
a: \(\dfrac{a}{a+b}=\dfrac{bk}{bk+b}=\dfrac{k}{k+1}\)
\(\dfrac{c}{c+d}=\dfrac{dk}{dk+d}=\dfrac{k}{k+1}\)
Do đó: \(\dfrac{a}{a+b}=\dfrac{c}{c+d}\)
b: \(\dfrac{7a^2+5ac}{7a^2-5ac}=\dfrac{7\cdot b^2k^2+5\cdot bk\cdot dk}{7\cdot b^2k^2-5\cdot bk\cdot dk}\)
\(=\dfrac{7b^2k^2+5bdk^2}{7b^2k^2-5bdk^2}=\dfrac{7b^2+5bd}{7b^2-5bd}\)(đpcm)
Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=t\) \(\Rightarrow a=bt\);\(c=dt\)
rồi bạn thế vào điều phải chứng minh là ra
Ta có \(\frac{a}{c}=\frac{c}{b}\)
=> c2 = ab
a) Ta có: \(\frac{a^{2} + c^{2}}{b^{2} + c^{2}}= \frac{a^{2} + ab}{b^{2} + ab}\) = \(\frac{a\left ( a + b \right )}{b(a + b)}= \frac{a}{b}\) (đpcm)
b) Ta có: \(\frac{b^{2} - a^{2}}{a^{2} + c^{2}}= \frac{\left ( b - a \right )\left ( b + a \right )}{a^{2} + ab}= \frac{\left ( b - a \right )\left ( b + a \right )}{a\left ( b + a \right )}= \frac{b - a}{a}\) (đpcm)
b2 - a2 = (b - a)(b + a) đây là HĐT nhé có dạng: A2 - B2 = (A - B)(A + B)
a, Ta có :
\(\dfrac{a}{c}=\dfrac{c}{b}=\dfrac{a^2}{c^2}=\dfrac{c^2}{b^2}=\dfrac{a\cdot c}{c\cdot b}=\dfrac{a}{b}\left(1\right)\)
Áp dụng tính chất của dãy tỉ số bằng nhau:
\(\dfrac{a}{c}=\dfrac{c}{b}=\dfrac{a^2}{c^2}=\dfrac{c^2}{b^2}=\dfrac{a^2+c^2}{c^2+b^2}\left(2\right)\)
Từ (1) ; (2)⇒ĐPCM
b, Theo bài ra ta có :\(\dfrac{a}{c}=\dfrac{c}{b}\Leftrightarrow ab=c^2\)
Thay vào biểu thức và áp dụng công thức (b-a)(b+a)=\(b^2-a^2\)
\(\dfrac{b^2-a^2}{a^2+c^2}=\dfrac{\left(b+a\right)\left(b-a\right)}{a^2+ab}=\dfrac{\left(b+a\right)\left(b-a\right)}{a\left(a+b\right)}=\dfrac{b-a}{a}\)
⇒ĐPCM
Đặt a/b=c/d=k
=>a=bk; c=dk
a: \(\dfrac{3a+5b}{3a-5b}=\dfrac{3bk+5b}{3bk-5b}=\dfrac{3k+5}{3k-5}\)
\(\dfrac{3c+5d}{3c-5d}=\dfrac{3dk+5d}{3dk-5d}=\dfrac{3k+5}{3k-5}\)
Do đó: \(\dfrac{3a+5b}{3a-5b}=\dfrac{3c+5d}{3c-5d}\)
b: \(\left(\dfrac{a+b}{c+d}\right)^2=\left(\dfrac{bk+b}{dk+d}\right)^2=\left(\dfrac{b}{d}\right)^2\)
\(\dfrac{a^2+b^2}{c^2+d^2}=\dfrac{b^2k^2+b^2}{d^2k^2+d^2}=\dfrac{b^2}{d^2}\)
Do đó: \(\left(\dfrac{a+b}{c+d}\right)^2=\dfrac{a^2+b^2}{c^2+d^2}\)
c: \(\dfrac{a-b}{a+b}=\dfrac{bk-b}{bk+b}=\dfrac{k-1}{k+1}\)
\(\dfrac{c-d}{c+d}=\dfrac{dk-d}{dk+d}=\dfrac{k-1}{k+1}\)
Do đó: \(\dfrac{a-b}{a+b}=\dfrac{c-d}{c+d}\)
a: Đặt a/b=c/d=k
=>a=bk; c=dk
\(\dfrac{a}{a-b}=\dfrac{bk}{bk-b}=\dfrac{k}{k-1}\)
\(\dfrac{c}{c-d}=\dfrac{dk}{dk-d}=\dfrac{k}{k-1}\)
Do đó: \(\dfrac{a}{a-b}=\dfrac{c}{c-d}\)
b: Đặt a/b=c/d=k
=>a=bk; c=dk
\(\left(\dfrac{a+b}{c+d}\right)^2=\left(\dfrac{bk+b}{dk+d}\right)^2=\dfrac{b^2}{d^2}\)
\(\dfrac{a^2+b^2}{c^2+d^2}=\dfrac{b^2k^2+b^2}{d^2k^2+d^2}=\dfrac{b^2}{d^2}\)
DO đó: \(\left(\dfrac{a+b}{c+d}\right)^2=\dfrac{a^2+b^2}{c^2+d^2}\)
Câu 1:
Đặt a/b=c/d=k
=>a=bk; c=dk
a: \(\dfrac{a^2}{c^2}=\dfrac{b^2k^2}{d^2k^2}=\dfrac{b^2}{d^2}\)
\(\dfrac{2a^2+3b^2}{2c^2+3d^2}=\dfrac{2b^2k^2+3b^2}{2d^2k^2+3d^2}=\dfrac{b^2}{d^2}\)
=>\(\dfrac{a^2}{c^2}=\dfrac{2a^2+3b^2}{2c^2+3d^2}\)
b: \(\dfrac{2a-3c}{c}=\dfrac{2bk-3dk}{dk}=\dfrac{2b-3d}{d}\)
a: a/b=c/d=k
=>a=bk; c=dk
\(\dfrac{a}{a-b}=\dfrac{bk}{bk-b}=\dfrac{k}{k-1}\)
\(\dfrac{c}{c-d}=\dfrac{dk}{dk-d}=\dfrac{k}{k-1}=\dfrac{a}{a-b}\)
b: \(\dfrac{a}{b}=\dfrac{bk}{b}=k\)
\(\dfrac{a+c}{b+d}=\dfrac{bk+dk}{b+d}=k=\dfrac{a}{b}\)
c \(\dfrac{a}{3a+b}=\dfrac{bk}{3bk+b}=\dfrac{k}{3k+1}\)
\(\dfrac{c}{3c+d}=\dfrac{dk}{3dk+d}=\dfrac{k}{3k+1}=\dfrac{a}{3a+b}\)
d: \(\dfrac{ac}{bd}=\dfrac{bk\cdot dk}{bd}=k^2\)
\(\dfrac{a^2+c^2}{b^2+d^2}=\dfrac{b^2k^2+d^2k^2}{b^2+d^2}=k^2=\dfrac{ac}{bd}\)
mình ko biết đúng hay sai các bạn nếu thấy mình sai thì các bạn sửa hộ mình
ta có \(\dfrac{a}{c}=\dfrac{c}{b}=>ab=c^2\)
=>\(\dfrac{b^2-a^2}{a^2+c^2}=\dfrac{b^2-ab+ab-a^2}{a^2+ab}=\dfrac{b.\left(b-a\right)+a.\left(b-a\right)}{a.\left(a+b\right)}=\dfrac{\left(b+a\right).\left(b-a\right)}{a.\left(a+b\right)}=\dfrac{b-a}{a}=>ĐPCM\)
Bạn làm đúng rùi đó,nhưng tự hỏi tự trả lời à???