ab - ac + bc - c² = -1

=> a.(b - c) + c.(b - c) = -1

=> (b - c) . (a + c) = -1

=> (b - c) . (a + c) = -1.1 = 1.(-1)

+) b - c = -1 thì a + c = 1

=> b - c + a + c = a + b = -1 + 1 = 0

+) b - c = 1 thì a + c = -1

=> b - c + a + c = a + b = 1 + (-1) = 0

Vậy a + b = 0.

a) Ta có:

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

Theo tc dãy tỉ số bằng nhau ta có:

 \(\frac{7a^2}{7b^2}=\frac{5ac}{5bd}=\frac{7a^2+5ac}{7b^2+5bd}=\frac{7a^2-5ac}{7b^2-5bd}\)

=>\(\frac{7a^2+5ac}{7a^2-5ac}=\frac{7b^2+5ad}{7b^2-5ad}\)

2+2+2+2=8 quả táo

Tick mk nhá.