Ridicam la patrat cele doua egalitati.
[tex](sina+cosb)^2=1^2=>sin^2a+2sina\cdot cosb+cos^2b=1[1]\\
(cosa+sinb)^2= (\frac{1}{2})^2=>cos^2a+2cos\ \cdot sinb+sin^2b= \frac{1}{4}[2]\\
Adunam \ [1]+[2]=> 1+2sin(a+b)+1= \frac{5}{4} \\
2sin(a+b)= \frac{5}{4} -2\\
2sin(a+b)= -\frac{3}{4} \\
sin(a+b)= -\frac{3}{8} \\[/tex]
