Question

Daca a supra b esteegal cu 3 supra 7 atunci 3a+b supra a+3b

Answer 1

a/b = 3/7  ⇔  a/3 = b/7 = k   ⇒  a=3k si b=7k !
                                                           ⇵
                      (3a+b)/(a+3b) = (9k+7k)/(3k+21k) = 16k / 24k = 2/3 .

Answer 2

$\frac{a}{b} = \frac{3}{7} \\ = > 7a = 3b \\ \frac{3a + b}{a + 3b} = ?$
Inlocuiesti 3b de la numitor cu 7a

$\frac{3a + b}{a + 7a} | \: \: \times 3 \\ = > \frac{3(3a + b)}{3 \times 8a} = \frac{9a + 3b}{24a} = \frac{9a + 7a}{24a} = \frac{16a}{24a} = \frac{16}{24} = \frac{2}{3}$