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Practice Spotting Calculation Errors

9.7.4 Practice Spotting Calculation Errors

9.7.4 • Practice Spotting Calculation Errors

Cool Down

1.

Here is someone's solution to the equation 3x2−4x=203x2−4x=20.

Step 1 - 3x2−4x=203x2−4x=20

Step 2 - a=3a=3, b=−4b=−4, c=20c=20

Step 3 - x=−b±b2−4ac2ax=−b±b2−4ac2a

Step 4 - x=−4±(−4)2−(4)(3)(20)2(3)x=−4±(−4)2−(4)(3)(20)2(3)

Step 5 - x=−4±−16−2406x=−4±−16−2406

Step 6 - x=−4±−2566x=−4±−2566

No Solutions

Identify as many errors as you can and briefly explain each error.

Compare your answer:

  • bb is -4, so in Step 4, the value −b−b should be -(-4), which is 4.
  • In Step 1, the equation should be rewritten as ax2+bx+c=0ax2+bx+c=0, so the constant term cc is -20, not 20. So, in Step 4, the last part of the numerator should be (−4)2−4(3)(−20)(−4)2−4(3)(−20).
  • In Step 4, (−4)2(−4)2 should be 16. Squaring a negative number gives a positive number.
  • If the earlier issues were fixed, then the values under the square root symbol would be 16+24016+240 or 256.