Warning: Illegal string offset 'html' in /home/hsn/public_html/forum/cache/skin_cache/cacheid_1/skin_topic.php on line 909

vectors? - HSN forum

# vectors?

### #1seren

Newbie

• Members
• 1 posts
• Gender:Female

Posted 19 May 2009 - 02:10 PM

question :the diagram shows vectors a and b if |a| = 5 , |b|=4 and a.(a+b)=36, find the size of the acute angle (feeta) between a andb

past paper 2003 paper 2 question 9

i know the formula to get cos(feeta) but dont know how to find a.b from those numbers, i know how i would normally get them....pleaase help

### #2SncZ

Site Swot

• Members
• 114 posts
• Location:Edinburgh
• Gender:Male

Posted 19 May 2009 - 02:37 PM

so we know: a . b = |a||b|cos(theta) and a.(a+b) =36

if we expand a.(a+b) =36 we get (a.a)+(a.b) =36,

a.a = 5x5xcos(0) = 25 since cos(0) =1 so 25 + (a.b) =36 therefore a.b =11

a.b = |a||b|cos(theta) ---> 11 = 5 x 4 x cos(theta)

thefore 11=20cos(theta)

theta = inverse cos(11/20)

theta = 56

#### 1 user(s) are reading this topic

0 members, 1 guests, 0 anonymous users