B= cos^2 25 độ + cos^2 35 độ + cos^2 45 độ + cos^2 55 độ + cos^2 65 độ
2 câu trả lời
$B=cos^225° + cos^235° + cos^245° + cos^255° + cos^265°$
$=cos^225° + cos^235° + cos^245° + sin^235° + sin^225°$
$=cos^225° + sin^225° + sin^235° + cos^235° + cos^245°$
$=(cos^225° + sin^225°) + (sin^235° + cos^235°) + cos^245°$
$=1+1+cos^245°$
$=\dfrac{1}{2}+ 2$
$=\dfrac{5}{2}$
#andy
\[\begin{array}{l}
B = co{s^2}{25^o} + co{s^2}{35^o} + co{s^2}{45^2} + co{s^2}{55^o} + co{s^2}{65^o}\\
= co{s^2}{25^o} + co{s^2}{35^o} + co{s^2}{45^o} + si{n^2}{35^o} + si{n^2}{25^o}\\
= co{s^2}{25^o} + si{n^2}{25^o} + si{n^2}{35^o} + co{s^2}{35^o} + co{s^2}{45^o}\\
= (co{s^2}{25^o} + si{n^2}{25^o}) + (si{n^2}{35^o} + co{s^2}{35^o}) + co{s^2}{45^o}\\
= 1 + 1 + co{s^2}{45^o}\\
= \dfrac{5}{2}
\end{array}\]
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