🚧 Avoid common mistakes · 1 min read
Watch your minus signs
Sign errors are the most common way good students lose easy marks.
The rules
Multiplying or dividing: same signs give a positive answer, different signs give a negative answer. (−3) × (−4) = 12, but (−3) × 4 = −12.
Subtracting a negative is the same as adding: 5 − (−2) = 5 + 2 = 7.
Where the mistakes happen
Squaring: (−5)2 = 25, but −52 = −25, because the square is done first and then the minus.
Expanding: −2(x − 3) = −2x + 6, not −2x − 6. The minus multiplies both terms.
The quadratic formula: if b = −4, then −b = 4 and b2 = 16.