Chọn đáp án C

a: VT=sin^2a(sin^2a+cos^2a)+cos^2a

=sin^2a+cos^2a

=1=VP

b: \(VT=\dfrac{sina+sina\cdot cosa+sina-sina\cdot cosa}{1-cos^2a}=\dfrac{2sina}{sin^2a}=\dfrac{2}{sina}=VP\)

c: \(VT=\dfrac{sin^2a+1+2cosa+cos^2a}{sina\left(1+cosa\right)}\)

\(=\dfrac{2\left(cosa+1\right)}{sina\left(1+cosa\right)}=\dfrac{2}{sina}=VP\)

Góc 2α =  A M H ^

a, Ta có:  sin 2 α = A H A M = 2 A H A M = 2 A B . A C B C 2 = 2 sin α . cos α

b,  1 + cos2α =  1 + H M A M = H C A M = 2 H C B C =  2 . A C 2 B C 2 = 2 cos 2 α

c, 1 – cos2α =  1 - H M A M = H B A M = 2 H B B C =  2 . A B 2 B C 2 = 2 sin 2 α

Chọn C sai

- Vì đẳng thức đúng phải là: cos β = sin(90o - β)

Đáp án đúng : D

a: \(1-\sin^2\alpha=cos^2\alpha\)

b: \(\left(1-cos\alpha\right)\left(1+cos\alpha\right)=1-cos^2\alpha=\sin^2\alpha\)

c: \(1+\sin^2\alpha+cos^2\alpha=1+1=2\)

d: \(\sin^4\alpha+cos^4\alpha+2\cdot\sin^2\alpha\cdot cos^2\alpha\)

\(=\left(\sin^2\alpha+cos^2\alpha\right)^2\)

=1^2=1

e: \(\tan^2\alpha-\sin^2\alpha\cdot\tan^2\alpha\)

\(=\tan^2\alpha\left(1-\sin^2\alpha\right)=\tan^2\alpha\cdot cos^2\alpha=\sin^2\alpha\)

a) Chọn C

b) Chọn C sai

- Vì đẳng thức đúng phải là:  cos   β   =   sin ( 90 °   -   β )

a: (sina+cosa)^2

=sin^2a+cos^2a+2*sina*cosa

=1+sin2a

b: \(cos^4a-sin^4a=\left(cos^2a-sin^2a\right)\left(cos^2a+sin^2a\right)\)

\(=cos^2a-sin^2a=cos2a\)