Posted by **Anonymous** on Saturday, March 29, 2008 at 7:18pm.

Find the dot product of the following vectors:

a = (4, 1) b = (-1,4)

a = (7,8) b = (4, -7)

You have to find the angle too.

Can someone please do these?

- Math -
**Damon**, Saturday, March 29, 2008 at 9:22pm
4*-1 + 1*4 = 0

cos angle = 0, so angle is 90 or -90 (240)

remember algebra 1

y = m x + b

the slope of a line perpendicular to that is -1/m

here we have a slope of 1/4

and the other slope is 4/-1

sure enough, perpendicular

7*4 + 8*-7

7*(4-8)= -28

|A|*|B|* cos angle = -28

sqrt(49+64)*sqrt(16+49)* cos angle = -28

10.6 * 8.06 *cos angle = -28

cos angle = -.328

angle = 109 deg

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