# Thread: proof: triangle of variable length on the Cartesian plane

1. ## proof: triangle of variable length on the Cartesian plane

A variable triangle OAB is formed by a straight line passing through the point P(a, b) on the Cartesian plane and cutting through the x-axis and y-axis at A and B respectively.
If the angle OAB=theta, find the area of triangle OAB in terms of a, b, and theta.

2. Originally Posted by shawli
A variable triangle OAB is formed by a straight line passing through the point P(a, b) on the Cartesian plane and cutting through the x-axis and y-axis at A and B respectively.
If the angle OAB=theta, find the area of triangle OAB in terms of a, b, and theta.
Hello Shawli:

grad $=tan(\theta)$

$tan(\theta)=\frac{b-B}{a}$

$b-a.tan(\theta)=B$

$tan(\theta)=\frac{b}{a-A}$

$\frac{a.tan(\theta)-b}{tan(\theta)}=A$

$Area=\frac{1}{2}.A.B$ (Substitute the values of A & B)

Hope this helps

3. Name the points $B0,c)~\&~Ad,0)" alt="B0,c)~\&~Ad,0)" />.
Then the area of the triangle is $\frac{cd}{2}$.
The slope of the line is $-\tan(\theta)$.
Use that to find $c~\&~d$.