2024 AMC 8

Complete problem set with solutions and individual problem pages

Problem 11 Medium

The coordinates of \triangle ABC are A(5,7), B(11,7), and C(3,y), with y>7. The area of \triangle ABC is 12. What is the value of y?

  • A.

    8

  • B.

    9

  • C.

    10

  • D.

    11

  • E.

    12

Answer:D

Solution 1

Since the triangle has a base of 6, we can plug in that value as the base. Then, we can solve the equation for the height. Doing so gives us,

\dfrac{6h}{2}=3h=12.

This means that h=4, so that means that we have to add 4 to the y-coordinate. So the answer is 7+4=\boxed{(D) 11}

 

Solution 2

By the Shoelace Theorem, \triangle ABC has area

\frac{1}{2}|(y \cdot 11 + 7 \cdot 5 + 7 \cdot 3) - (3 \cdot 7 + 11 \cdot 7 + 5 \cdot y)| = \frac{1}{2}|(11y + 56) - (98 + 5y)| = \frac{1}{2}|6y - 42|.

From the problem, this is equal to 12. We now solve for y.

\frac{1}{2}|6y - 42| = 12

|6y-42| = 24

6y - 42 = 24 OR 6y - 42 = -24

6y = 66 OR 6y = 18

y = 11 OR y = 3

However, since, as stated in the problem, y > 7, our only valid solution is \boxed{\textbf{(D)} 11}.