1. Cho tỉ lệ thức: \(\frac{a}{b}=\frac{c}{d}\). CMR: \(\frac{2a^2-3ab+4b^2}{5b^2+6ab}=\frac{2c^2-3cd+4d^2}{5d^2+6cd}\)
2. Cho \(\frac{a}{b}=\frac{c}{d}\). CMR:
a. \(\frac{a^2+b^2}{b^2+c^2}=\frac{a}{c}\)
b. \(\frac{c^2-a^2}{a^2+b^2}=\frac{c-a}{a}\)