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(sinA+sinB)^2+(cosA+cosB)^2=4cos^2 (A-B/2)

Răspuns :

sinA^2+sinB^2+2*sinA*sinB+cosA^2+cosB^2+2*cosA*cosB=
= sinA^2+cosA^2+sinB^2+cosB^2 + 2(sinA*sinB+cosA*cosB)
= 1 + 1 +2*cos(A-B)
=2 + 2*cos(2*
[tex] \frac{A-B}{2} [/tex])
=2+ 2(2 cos^2(
[tex] \frac{A-B}{2} [/tex])-1)
=2+ 4 cos^2
([tex] \frac{A-B}{2} [/tex]) -2
=
4 cos^2([tex] \frac{A-B}{2} [/tex])