Advanced MathHardNumeric check of the cube-of-a-sum identity
Advanced Math practice question
If a = 3 and b = −2, what is the value of (a + b)³ − (a³ + b³)?
Answer: -18
(a + b)³ = 1 and a³ + b³ = 27 − 8 = 19, so the difference is 1 − 19 = −18. (In general it equals 3ab(a + b) = 3(−6)(1).)
Common wrong answers
- If you answered 0
- 0 assumes (a + b)³ = a³ + b³.
- If you answered 18
- 18 has the wrong sign.
- If you answered -34
- −34 takes b³ as +8.
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