Question
If $(1 + \tan \theta )(1 + \tan \phi ) = 2$, then $\theta + \phi =$ ....$^o$
$\Rightarrow \frac{{\tan \theta + \tan \phi }}{{1 - \tan \theta \tan \phi }} = 1$
$ \Rightarrow $ $\tan (\theta + \phi ) = 1$
$ \Rightarrow $ $\theta + \phi = \frac{\pi }{4} = 45^\circ$.
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$1 + 6 + \frac{{9({1^2} + {2^2} + {3^2})}}{7} + \frac{{12({1^2} + {2^2} + {3^2} + {4^2})}}{9} + \frac{{15({1^2} + {2^2} + .... + {5^2})}}{{11}} + ...$ up to $15$ terms, is