Question

7x + 3y = 15 ; 12y - 5x = 39

Answer 1

7x + 3y = 15 ----------------  [ 1 ]

12y - 5x = 39 ----------------  [ 2 ]

[ 1 } x 4: 28x + 12y = 60 ----------------  [ 3 ]

equation [ 3 ]  - [ 2 ]:

(28x + 12y) - (12y - 5x) = 60 - 39

28x + 12y - 12y + 5x = 60 - 39

33x = 21

x = 21/33

x = 7/11

Find the value of y:

7x + 3y = 15 ----------------  [ 1 ]

Sub x = 7/11 :

7 (7/11) + 3y = 15

3y = 15 - 49/11

3y = 116/11

y = 1116/11 ÷ 3

y = 116/33

Answer: The value of x = 7/11 and the value of y is 116/33

Answer 2

7x + 3y = 15

7x + 3y -15 =0..(1)

a1 = 7 ; b1=3 ;c1 = -15


12y - 5x = 39

-5x +12y -39 = 0....(2)

a2 = -5 ; b2=12 ;c2 = -39



Cross Multiplication method..=>

b1 c1 a1 b1
b2 c2 a2 b2

x/(b1c2 - b2c1) = y/(c1a2 -a1c2)= 1/(a1b2-a2b1)

3 -15 7 3
12 -39 -5 12

substituting values...

x/((3×-39)- (-15×12)) = y/((-15×-5)- (-39×7)) = 1/((7×12)-(-5×3))

x/(-117+180) = y/(-75+273) = 1/(84 + 15)

x/(63) = y/(198) = 1/(99)

x= 63/99 = 7/11 ; y = 198/99 = 2.