Tính $C$$\frac{1}{cosa}+1=\frac{1+cosa}{cosa}=\frac{2cos^2\frac{a}{2}}{cosa}$
$\frac{1}{cos2a}+1=\frac{1+cos2a}{cos2a}=\frac{2cos^2a}{cos2a}$
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$\frac{1}{cos{2^na}}+1=\frac{1+cos{2^na}}{cos{2^na}}=\frac{2cos^2{2^{n-1}a}}{cos{2^na}}$
$\Rightarrow C=2^{n+1}.cos^2\frac{a}{2}.cosa.cos{2a}......cos{2^{n-1}a}.\frac{1}{cos{2^na}}$
$=2^{n+1}.cos\frac{a}{2}.\frac{1}{cos{2^na}}.(cos\frac{a}{2}.cosa......cos{2^{n-1}a})$
Áp dụng công thức tính $B$ ta có
$C=2^{n+1}.cos\frac{a}{2}.\frac{1}{cos{2^na}}.\frac{1}{2^{n+1}}.\frac{sin{2^na}}{sin\frac{a}{2}}$
$=cot\frac{a}{2}.tan{2^na}$