The value of $\sum_{r-1}^{18} cos^2(5r)^o,$ where $x^o $ denotes the $x$ degree, is equals to
The value of $\sum_{r-1}^{18} cos^2(5r)^o,$ where $x^o $ denotes the $x$ degree, is equals to
- A
$\frac{19}{2}$
- B
$\frac{7}{2}$
- C
$\frac{17}{2}$
- D
$0$
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