245sin

Bạn giải từng bước hộ mik đc ko

mik hay milk

\(B=\sin^247^o\times\cos45^o+\sin45^o\times\cos^247^o\)
\(B=\sin^247^o\times\cos45^o+\cos45^o\times\cos^247^o\)
\(B=\cos45^o\left(\sin^247^o+\cos^247^o\right)\)
\(B=\cos45^o.1=\cos45^o\)

\(gi\text{ỏi}-qu\text{á}-nh\text{ỉ}\)

Thanks :) <3

\(=2008\left(\sin^220^o+\cos^220^o\right)+\cos70^o-\cos70^o+\frac{\sin20^o}{\cos20}.\frac{sin70}{c\text{os}70}\)

\(=2008+1=2009\)

oh , bác sĩ ơi tui sắp chết

Chú ý 2 điều: \(\cos45^o=\sin45^o=\frac{\sqrt{2}}{2}\) và \(\cos^2a+\sin^2a=1\)

Do đó: 

a) \(A=\cos^252^o.\frac{\sqrt{2}}{2}+\sin^252^o.\frac{\sqrt{2}}{2}=\frac{\sqrt{2}}{2}\left(\cos^252^o+\sin^252^o\right)=\frac{\sqrt{2}}{2}.1=\frac{\sqrt{2}}{2}\)

b) \(B=\frac{\sqrt{2}}{2}.\cos^247^o+\frac{\sqrt{2}}{2}.\sin^247^o=\frac{\sqrt{2}}{2}\left(\cos^247^o+\sin^247^o\right)=\frac{\sqrt{2}}{2}.1=\frac{\sqrt{2}}{2}\)

\(=\left(1-sin^247^0\right)\cdot sin45^0+sin47^0\cdot\left(1-sin^245^0\right)\)

\(=sin45-sin^247^0\cdot\dfrac{\sqrt{2}}{2}+sin47^0-\dfrac{1}{2}\cdot sin47^0\)

\(\simeq0.69\)