A triangle has corners at (6 ,2 )(6,2), (5 ,-8 )(5,8), and (-5 ,3 )(5,3). If the triangle is dilated by a factor of 5 5 about point #(7 ,-2 ), how far will its centroid move?

1 Answer
Oct 10, 2016

The distance moved is 4sqrt(26)426

Explanation:

Let's begin by computing the current centroid, point O_1O1,

O_(1x) = (6 + 5 - 5)/3 = 2O1x=6+553=2

O_(1y) = (2 - 8 + 3)/3 = -1O1y=28+33=1

Point O_1 = (2, -1)O1=(2,1)

  1. Scale point (6, 2)(6,2):

Compute the vector from point (7, -2)(7,2) to point (6, 2)(6,2):

(6 - 7)hati + (2 - -2)hatj = -hati + 4hatj(67)ˆi+(22)ˆj=ˆi+4ˆj

Scale by 5:

-5hati + 20hatj5ˆi+20ˆj

Find the new end point:

(-5+ 7, 20 + -2) = (2, 18)(5+7,20+2)=(2,18)

  1. Scale point (5, -8)(5,8):

Compute the vector from point (7, -2)(7,2) to point (5, -8)(5,8):

(5 - 7)hati + (-8 - -2)hatj = -2hati -6hatj(57)ˆi+(82)ˆj=2ˆi6ˆj

Scale by 5:

-10hati - 30hatj10ˆi30ˆj

Find the new end point:

(-10+ 7, -30 + -2) = (-3, -32)(10+7,30+2)=(3,32)

  1. Scale point (-5, 3)(5,3):

Compute the vector from point (7, -2)(7,2) to point (-5, 3)(5,3):

(-5 - 7)hati + (3 - -2)hatj = -12hati + 5hatj(57)ˆi+(32)ˆj=12ˆi+5ˆj

Scale by 5:

-60hati + 25hatj60ˆi+25ˆj

Find the new end point:

(-60+ 7, 25 + -2) = (-53, 23)(60+7,25+2)=(53,23)

Our scaled triangle has the vertices, (2, 18)(2,18), (-3, -32)(3,32), and (-53, 23)(53,23). The new centroid, O_2O2, has coordinates:

O_(2x) = (2 - 3 - 53)/3 = -18O2x=23533=18

O_(2y) = (18 - 32 + 23)/3 = 3O2y=1832+233=3

O_2 = (-18, 3)O2=(18,3)

Compute the distance, d, between O_1O1 and O_2O2:

d = sqrt((-18 - 2)^2 + (3 - -1)^2)d=(182)2+(31)2

d = sqrt((-20)^2 + 4^2)d=(20)2+42

d = sqrt((-20)^2 + 4^2)d=(20)2+42

d = sqrt(416)d=416

d = 4sqrt(26)d=426