AlgebraHardThird point on a line
Algebra practice question
In the xy-plane, the points (1, 2), (3, 8) and (5, k) lie on the same line. What is the value of k?
Answer: 14
The slope from (1, 2) to (3, 8) is (8 − 2)/(3 − 1) = 3. From (3, 8), moving 2 more in x raises y by 6: k = 8 + 6 = 14.
Common wrong answers
- If you answered 10
- 10 is 8 + 2, adding the change in x instead of the change in y.
- If you answered 11
- 11 adds the slope once (8 + 3), but x increases by 2 from 3 to 5.
- If you answered 16
- 16 is 2 × 8, doubling the middle point's y-value.
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