Question #fca5d

1 Answer
Apr 17, 2017

Answer:

#"theta=pi/2#

Explanation:

#"you can use the cosine law."#
#r^2=a^2+b^2+2ab*cos theta#

#15^2=9^2+12^2+2*15*9*cos theta#

#225=81+144+270*cos theta#

#225=225+270*cos theta#

#225-225=270*cos theta#

#0=270*cos theta#

#cos theta =0#

#arc cos( 0)=pi/2#

#theta=pi/2#