Răspuns:
Obs!! Relatiile folosite: Sinusul si cosinusul sunt complementare, formule de reducere la primul cadran.
[tex]sin(40)*sin(140)=cos^2(130)[/tex]
[tex]sin(40)=cos(90-40)=cos(50)\\[/tex]
[tex]sin(140)=cos(90-140)=cos(-50)=cos(50)[/tex]
[tex]cos^2(130)=cos^2(180-130)=cos^2(50)[/tex]
Inlocuim:
[tex]sin(40)*sin(140)=cos(50)*cos(50)=cos^2(50)[/tex] "Adevarat"
