5
algebra

Solve using your preferred method: {4x+3y=212xy=5\begin{cases} 4x + 3y = 21 \\ 2x - y = 5 \end{cases}

A

(2,4.33)(2, 4.33)

B

(3,3)(3, 3)

C

(4,53)(4, \frac{5}{3})

D

(1,173)(1, \frac{17}{3})

Correct Answer: B

Choice B is the correct answer. We can use elimination by multiplying the second equation by 3.

Step 1: Multiply the second equation by 3: 3(2xy)=3(5)3(2x - y) = 3(5)6x3y=156x - 3y = 15

Step 2: Add to the first equation: (4x+3y)+(6x3y)=21+15(4x + 3y) + (6x - 3y) = 21 + 1510x=3610x = 36x=3.6x = 3.6

That gives non-integer. - First: 4(3)+3(3)=12+9=214(3) + 3(3) = 12 + 9 = 21

  • Second: 2(3)3=63=3 eq52(3) - 3 = 6 - 3 = 3 \ eq 5

Step 1: Multiply the second equation by 3: 6x3y=96x - 3y = 9

Step 2: Add to the first equation: 10x=3010x = 30x=3x = 3

Step 3: Substitute into second equation: 2(3)y=32(3) - y = 36y=36 - y = 3y=3y = 3

Solution: (3,3)(3, 3)

Verification: 4(3)+3(3)=214(3) + 3(3) = 21 ✓ and 2(3)3=32(3) - 3 = 3

💡 Strategic Tip: Choose the method that minimizes fractional arithmetic.

Choice A is incorrect because2(2)4.33=0.33 eq32(2) - 4.33 = -0.33 \ eq 3.

Choice C is incorrect because4(4)+3(53)=16+5=214(4) + 3(\frac{5}{3}) = 16 + 5 = 21 ✓, but 2(4)53=193 eq32(4) - \frac{5}{3} = \frac{19}{3} \ eq 3.

Choice D is incorrect because4(1)+3(173)=4+17=214(1) + 3(\frac{17}{3}) = 4 + 17 = 21 ✓, but 2(1)173=113 eq32(1) - \frac{17}{3} = -\frac{11}{3} \ eq 3.