Question

Fie a si b direct proporționale cu {3,4} afla nr daca a+b=21

Answer 1

a/3=b/4

4a=3b

a+b=21

a+4a/3=21

aducem la acelasi numitor

3a/3+4a/3=21*3/3

3a+4a=63

7a=63

a=9

b=21-9

b=12

Answer 2

[a; b ] d. p.  [ 3; 4]

a / 3 = b / 4 = k ⇒  a = 3 k    si    b = 4 k

a + b = 21

3 k + 4 k = 21

7 k = 21  ⇒  k = 21 : 7 ⇒  k = 3

a = 3 × 3 = 9

b = 4 × 3 = 12

V:   9 + 12 = 21 ;   9 / 3 = 12 / 4 = 3