If 7x = 8y, then x: y=______ ：______ ．

If 7x = 8y, then x: y=______ ：______ ．

Because 7x = 8y, so x: y = 8:7

x-y=13 x=6y-7

Substitute formula 2 into Formula 1 to get 6y-7-y = 13
5y=13+7
y=4
x=13+4=17

Solve the following equations: (1) x − y = 13X = 6y − 7; (2) 5x + 2Y = 253x + 4Y = 15

(1) X − y = 13, ① x = 6y − 7, ②, 6y-7-y = 13, y = 4, y = 17, x = 17Y = 4; (2) 5x + 2Y = 25, ① 3x + 4Y = 15, ② × 2 - ②, 7x = 35, x = 5, ② × 5 - ① × 3, 14y = 0, y = 0, x = 5Y = 0

Calculation: 1. (- 6y + x) (- 6y-x) 2. (- 6y + x) (6y-x) 3. (x + 4Y) (x-9y) 4. (- 6y-x) (6y-x)

1.(-6y+x)(-6y-x) =[-(6y-x)][-(6y+x)] =(6y-x)(6y+x) =(6y)²-x² =36y²-x² 2.(-6y+x)(6y-x) =[-(6y-x)](6y-x) =-(6y-x)(6y-x) =-[(6y-x)²] =-（36y²-12xy-x²） =x²+12xy-36...