Maths Olympiad Prep

Track / Stage 3 / 209 of 260 #209 of 1964

Problem 209

AMC 10/12, early questions
Algebra Difficulty 3.6 Multiple choice

Given the vectors a=(x3,2) \overrightarrow{a}=(x-3,2) and b=(1,1) \overrightarrow{b}=(1,1) , the statement " x>1 x > 1 " is a ______ condition for "the angle between a \overrightarrow{a} and b \overrightarrow{b} is acute".

Pick one

Official solution

Let's calculate the dot product of vectors a \overrightarrow{a} and b \overrightarrow{b} :

ab=(x3)1+21=x1 \overrightarrow{a} \cdot \overrightarrow{b} = (x-3) \cdot 1 + 2 \cdot 1 = x - 1

For the vectors a \overrightarrow{a} and b \overrightarrow{b} to be collinear with the same direction, we must have b=λa \overrightarrow{b} = \lambda \overrightarrow{a} , with λ>0 \lambda > 0 .

Setting up the equations:

{x3=λ2=λ \begin{cases} x - 3 = \lambda \\ 2 = \lambda \end{cases}

we find that x=5 x = 5 . When x=5 x = 5 , the condition x>1 x > 1 is satisfied, but in this case, the angle between the two vectors is 0 0^{\circ} , so the angle being acute is not true.

If a \overrightarrow{a} and b \overrightarrow{b} form an acute angle, then their dot product must be positive:

ab=x1>0 \overrightarrow{a} \cdot \overrightarrow{b} = x - 1 > 0

Thus, x>1 x > 1 must be true. This implies that " x>1 x > 1 " is a necessary but not sufficient condition for the angle between a \overrightarrow{a} and b \overrightarrow{b} to be acute.

Therefore, the correct answer is:
A \boxed{ A }

This problem assesses the understanding of necessary and sufficient conditions along with the relationship between the angle of vectors and their dot product.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.