# Homework Help: Abstract vector basis

1. Mar 31, 2015

### PcumP_Ravenclaw

1. The problem statement, all variables and given/known data
Suppose that $u = s_1i + s_2j$ and $v = t_1i + t_2j$, where s1, s2, t1 and t2 are real
numbers. Find a necessary and sufficient condition on these real numbers
such that every vector in the plane of i and j can be expressed as a linear
combination of the vectors u and v.

2. Relevant equations

We shall need to consider directed line segments, and we denote the directed
line segment from the point a to the point b by [a, b]. Specifically, [a, b] is the
set of points {a + t(b − a) : 0 ≤ t ≤ 1},

3. The attempt at a solution
As attached.

#### Attached Files:

• ###### q3_4.1.jpg
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2. Mar 31, 2015

### BvU

Phrase it this way: if $\vec w = x \; \hat\imath + y \; \hat \jmath$ then what do you have to do to write it as $\vec w = a \; \vec u + b\; \vec v$ ?
When can you do that and when can you not do that ?

3. Mar 31, 2015

### PcumP_Ravenclaw

$\vec u = s_1 \vec i+ s_2 \vec j$

$\vec v = t_1 \vec i+ t_2 \vec j$

$x = a s_1 + b t_1$

$y = a s_2 + b t_2$

4. Mar 31, 2015

### Staff: Mentor

Is it always possible to solve for a and b? What if u or v is the zero vector? What if u and v are equal? What if u is a nonzero multiple of v?

You have a very simple space here -- the plane. The question boils down to this: what does it take for two vectors to span a plane?

5. Apr 1, 2015

### BvU

So, try to solve for a and b and see what you get !

6. Apr 6, 2015

### PcumP_Ravenclaw

I believe I have only found two/three cases that restrict the coefficients of u and v. I have attached my solution.

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7. Apr 6, 2015

### BvU

Solving for a and b means you express the unknowns a and b in terms of the knowns, x, y, s1, s2, t1 and t2.