Kako prikazujete cos (x + pi / 2) + cos (x-pi / 2) = 0?

Uporabiti moramo identiteto trigonometrije:

#cos (A + -B) = cosAcosB sinAsinB #

S tem dobimo:

#cos (x + pi / 2) + cos (x-pi / 2) = (cosxcos (pi / 2) + sinxsin (pi / 2)) + (cosxcos (pi / 2) -sinxsin (pi / 2)) #

#cos (pi / 2) = 0 #

#sin (pi / 2) = 1 #

#cos (x + pi / 2) + cos (x-pi / 2) = (0cosx + 1sinx) + (0cosx-1sinx) = sinx-sinx = 0 #

Pokažite, da cos²π / 10 + cos²4π / 10 + cos² 6π / 10 + cos²9π / 10 = 2. Malo sem zmeden, če naredim Cos²4π / 10 = cos² (π-6π / 10) & cos²9π / 10 = cos² (π-π / 10), bo postal negativen kot cos (180 ° - theta) = - costheta v drugi kvadrant. Kako naj dokazujem vprašanje?

Glej spodaj. LHS = cos ^ 2 (pi / 10) + cos ^ 2 ((4pi) / 10) + cos ^ 2 ((6pi) / 10) + cos ^ 2 ((9pi) / 10) = cos ^ 2 (pi / 10) + cos ^ 2 ((4pi) / 10) + cos ^ 2 (pi- (4pi) / 10) + cos ^ 2 (pi- (pi) / 10) = cos ^ 2 (pi / 10) + cos ^ 2 ((4pi) / 10) + cos ^ 2 (pi / 10) + cos ^ 2 ((4pi) / 10) = 2 * [cos ^ 2 (pi / 10) + cos ^ 2 ((4pi) / 10)] = 2 * [cos ^ 2 (pi / 2- (4pi) / 10) + cos ^ 2 ((4pi) / 10)] = 2 * [sin ^ 2 ((4pi) / 10) + cos ^ 2 ((4pi) / 10)] = 2 * 1 = 2 = RHS

Kako dokazujete, da sqrt (3) cos (x + pi / 6) - cos (x + pi / 3) = cos (x) -sqrt3sinx?

LHS = sqrt3cos (x + pi / 6) -cos (x-pi / 3) = sqrt3 [cosx * cos (pi / 6) -sinx * sin (pi / 6)] - [cosx * cos (pi / 3) -sinx * sin (pi / 3)] = sqrt3 [cosx * (sqrt3 / 2) -sinx * (1/2)] - [cosx * (1/2) -sinx * (sqrt3 / 2)] = (3cosx -sqrt3sinx) / 2- (cosx-sqrt3sinx) / 2 = (3cosx-sqrt3sinx-cosx + sqrt3sinx) / 2 = (2cosx) / 2 = cosx = RHS

Kako preverite [sin ^ 3 (B) + cos ^ 3 (B)] / [sin (B) + cos (B)] = 1-sin (B) cos (B)?

Dokaz pod Ekspanzijo ^ 3 + b ^ 3 = (a + b) (^ 2-ab + b ^ 2) in lahko uporabimo to: (sin ^ 3B + cos ^ 3B) / (sinB + cosB) = ((sinB + cosB) (sin ^ 2B-sinBcosB + cos ^ 2B)) / (sinB + cosB) = sin ^ 2B-sinBcosB + cos ^ 2B = sin ^ 2B + cos ^ 2B-sinBcosB (identiteta: sin ^ 2x + cos ^ 2x = 1) = 1-sinBcosB