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Find the value of x and y in the simultaneous equation: 3x +...

Find the value of x and y in the simultaneous equation: 3x + y = 21; xy = 30

A.

x = 3 or 7, y = 12 or 8

B.

x = 6 or 1, y = 11 or 5

C.

x = 2 or 5, y = 15 or 6

D.

x = 1 or 5, y = 10 or 7

Correct answer is C

3x + y = 21 ... (i);

xy = 30 ... (ii)

From (ii), y=30x. Putting the value of y in (i), we have

3x + 30x = 21

3x2 + 30 = 21x

3x2 - 21x + 30 = 0

3x2 - 15x - 6x + 30 = 0

3x(x - 5) - 6(x - 5) = 0

(3x - 6)(x - 5) = 0

3x - 6 = 0 x = 2.

x - 5 = 0 x = 5.

If x = 2, y = 302 = 15;

If x = 5, y = 305 = 6.