S5 Equation of Straight Lines

If the two lines are parallel, their slope will be ____.

如果兩條線平行，它們的斜率會__。

line L1: y = 3x + 7. Find the slope of a straight line L2 that is parallel to L1.

直線L1：y = 3x + 7 。直線L2平行於L1。求L2的斜率。

line L1: -2x + y = 6. Find the slope of a straight line L2 that is parallel to L1.

直線L1：-2x + y = 6 。直線L2平行於L1。求L2的斜率。

Answer explanation

line L1: -2x + y = 6

By make y be the subject

y = 2x + 6

so the slope of L1 = 2

Since L1 and L2 are parallel, slope of L2 = slope of L1 = 2

If L1 and L2 are perpendicular, which one can describe their relation about slope?

如果 L1 和 L2 是垂直的，哪一個能描述它們在斜率上的關係？

If L1 and L2 are perpendicular, slope of L1 = 3, what is slope of L2?

如果 L1 和 L2 垂直，L1 的斜率 = 3，L2 的斜率是多少？

• 3

• -3

• 1/3

• -1/3

Answer explanation

slope of L1 x Slope of L2 = -1

3 x slope of L2 = -1

Slope of L2 = -1/3

If L1 and L2 are perpendicular, slope of L1 = 1/5, what is slope of L2?

如果 L1 和 L2 垂直，L1 的斜率 = 1/5，L2 的斜率是多少？

• 5

• -5

• 1/5

• -1/5

• 2/3

• -2/3

• 3/2

• -3/2

If the slope of straight line is 2 and the y intercept is -4, which form can you use to find the equation of straight line?

若直線斜率為2，y截距為-4，您可以使用哪種式來求直線方程?

• y = mx + c

If the slope of straight line is 5 and it passes through (4, 2), which form can you use to find the equation of straight line?

若直線斜率為5，它通過 (4, 2)，您可以使用哪種式來求直線方程?

• y = mx + c

Answer explanation

We don't have y-int, so we won't use y = mx + c.

Instead, we have the coordinate of a point, so we will use point slope form.

If the slope of straight line is 5 and the x intercept is -4, which form can you use to find the equation of straight line?

若直線斜率為5，x截距為-4，您可以使用哪種式來求直線方程?

• y = mx + c

Answer explanation

We can only use y = mx + c when we have y-int.

Because x intercept = -4, the coordinate of x intercept = (-4, 0)

We have the point, so we use point-slope form