Question 864456
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Hi
given sin a = 4/5 in quadrant I and cos ß = ???12/13 in <u>quadrant II</u> find cos a+ß 

given cos a = -24/25 in <u>quadrant II</u> &#8658; sin a = 7/25    {{{sqrt(25^2 - 24^2) = 7}}}
and sin ß = -5/13 in quadrant III   &#8658; cos ß = -12/13
cos(a + ß) = cos a cos ß  -  sin a sin ß     (substitute appropriate values)
           = (-24/25)(-12/13) - (7/25)(-5/13)   
           = (24*12+35)/25*13