Question

Cho \(\frac{a}{b}=\frac{c}{d}\)Chứng minh rằng:

a) \(\frac{\left(2a+3b\right)^2}{\left(3a-4b\right)^2}=\frac{\left(2c+3d\right)^2}{\left(3c-4d\right)^2}\)

b) \(\frac{2a^2-3ab+4b^2}{2b^2+5ab}=\frac{2c^2-3cd+4d^2}{2d^2+5cd}\)

Answer 1

mk làm câu a thôi, b dài nhưng tương tự

Gọi a/b=c/d=k =>a=bk ; c=dk

=>\(\frac{\left(2a+3b\right)^2}{\left(3a-4b\right)^2}=\frac{\left(2bk+3b\right)^2}{\left(3bk-4b\right)^2}=\frac{\left[b\left(2k+3\right)\right]^2}{\left[b\left(3k-4\right)\right]^2}=\frac{b^2\left(2k+3\right)^2}{b^2\left(3k-4\right)^2}=\frac{\left(2k+3\right)^2}{\left(3k-4\right)^2}\)(1)

=>\(\frac{\left(2c+3d\right)^2}{\left(3c-4d\right)^2}=\frac{\left(2dk+3d\right)^2}{\left(3dk-4d\right)^2}=\frac{\left[d\left(2k+3\right)\right]^2}{\left[d\left(3k-4\right)\right]^2}=\frac{\left(2k+3\right)^2}{\left(3k-4\right)^2}\)(2)

Từ (1);(2)=> đpcm