Question

Are the lines y = –x + 9 and 5x – 5y = 10 perpendicular? Explain.

Answer 1

yes because if you were to divide this equation by 5 and rearrange in for y, you can see that the slope is the negative reciprocal of the slope of the first
5x – 5y = 10
x – y = 2
y=x-2, slope=1 which is negative reciprocal of -1

Answer 2

The slope must be a negative reciprocal, you can see from this diagram below