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# C1 Geometry, should this way of substitution also work? watch

1. The first equation we were given was 2x + 3y = 26 for L1, and we needed to rearrange that in the first part of the question to find the gradient of L2 (perpendicular to L1). I rearranged it to get y = -2/3x +26/3, which was correct.

We could then work out the equation of L2, which was y=2/3x, which was correct.

However, for part B (this one in the image), you need to substitute them into each other to find where they intersect. ExamSolutions and MathsGenie solutions did it via using L1 in form ax+by+c and L2 in the form y=mx+c, replacing b with mx+c.

However, I tried doing it by using L1 in the form y=-2/3x +26/3 and L2 in form y=2/3x, made them equal to each other and tried it that way, so I had:
2/3x = -2/3x + 26/3

Are you not able to do it the way I tried? I tried it and got a horrible number when solving for x.

2. You have the equation of the second line wrong.
Your method is perfectly valid as actually is exactly he same method as the guy on the video - however you get the wrong answer because your equation of the second line is not right.
3. You have to reciprocate the gradient for the perpendicular line

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Updated: January 29, 2017
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