Bài 1:
cos (78) = sin ( 90 - 78) = sin (12)
cos(68) = sin ( 90 - 68 ) = sin (22)
cos (50) = sin ( 90-50) = sin (40)
Sắp xếp: sin (12) ; sin (22) ; sin (27) ; sin (29) ; sin (40) ; sin (64)
=> cos (78) ; cos(68) ; sin (27) ; sin (29) ; cos (50) ; sin (64)
Bài 2:
a) Xét ΔABC vuông tại A, ta có:
$BC^{2}$ = $AC^{2}$ + $AB^{2}$ ( định lý Py - ta - go)
=> $BC^{2}$ = $12^{2}$ + $5^{2}$ = 169
=> BC = 13 cm
Ta có: sin B = cos C = $\frac{AC}{BC}$ = $\frac{12}{13}$
=> Góc B = $67^{o}$
Góc C = Góc A - Góc B = $23^{o}$
b ) sin B = $\frac{AC}{BC}$ = $\frac{12}{13}$
cos B = $\frac{AB}{BC}$ = $\frac{5}{13}$
tg B = $\frac{AC}{AB}$ = $\frac{12}{5}$
cotg B = $\frac{AB}{AC}$ = $\frac{5}{12}$
