Question 1171626
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<pre>

For angle  " a "  in QII, 


    sin(a) = {{{sqrt(1-cos^2(a))}}} = {{{sqrt(1-(-4/5)^2)}}} = {{{sqrt(1-16/25)}}} = {{{sqrt(9/25)}}} = {{{3/5}}}  (positive value).



For angle  " b "  in QII, 


    cos(b) = - {{{sqrt(1-sin^2(b))}}} = - {{{sqrt(1-(5/13)^2)}}} = - {{{sqrt(1-25/169)}}} = - {{{sqrt(144/169)}}} = - {{{12/13}}}  (negative value).



Now use  cos(a+b) = cos(a)*cos(b) - sin(a)*sin(b).



Substitute the values into the formula and calculate.
</pre>

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To see many other similar solved problems on calculating trig functions, &nbsp;look into the lessons  

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Trigonometry-basics/Calculating-trigonometric-functions-of-angles.lesson>Calculating trigonometric functions of angles</A>

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Trigonometry-basics/Selected-problems-from-the-archive-on-calculating-trig-functions-of-angles.lesson>Advanced problems on calculating trigonometric functions of angles</A>

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Trigonometry-basics/Evaluating-trigonometric-expressions.lesson>Evaluating trigonometric expressions</A> 

in this site.


Also, &nbsp;you have this free of charge online textbook in ALGEBRA-II in this site

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson>ALGEBRA-II - YOUR ONLINE TEXTBOOK</A>.


The referred lessons are the part of this online textbook under the topic &nbsp;"<U>Trigonometry: Solved problems</U>". 



Save the link to this textbook together with its description


Free of charge online textbook in ALGEBRA-II
https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson


into your archive and use when it is needed.