- 278.
ա)+= 1
1++ = = = =
279.
բ) coscos (2) cos (4) cos (8) cos (16) =
= = = =
= = cos co2 cos cos
cos
280.
sin42 = 2sincos(2) = 4sin cos (cos-sin) = 4sin cos – 4sin cos
cos 4 = cos – sin(2) = (cos-sin) – (2sin cos) = cos + sin – 2sin
cos – 4sin cos = cos + sin – 6sin cos
281.
ա) sinA + sinB + sinC = 4cos* cos * cos
sinA + sinB + sinC = 2sin () cos () + sin () =
Քանի որ A+B = 180– C = 90– => sin () = cos
= 2cos () cos() + 2 sin () cos () = 2cos () (cos+ sin ) = 2cos () (cos+ sin
) = 2cos () (cos+ sin (90+)) = 2cos () (cos+ cos ) = 2cos () (2cos
() cos ()) = 2cos () (2coscos ()) = 4cos cos cos
282.
sin (45+) +sin (45–) = 2 sin () cos () = 2sin(45+ ) – sin (45– ) = 2cos () sin () = 2 cos 45sin = sin
= = ctg(
= = ctg( )