5 Questions around this concept.
In Triangle ABC, point D is on BC is such that $A D \perp B C$ and E is the middle point of BC and $b^2+2 a^2=c^2$ distance between D and E is?
Projection Formula
In the $\triangle A B C$,
The projection of $A B$ on $B C$ is $B D$ and the Projection of $A C$ on $B C$ is $D C$

Now,
$
\begin{aligned}
& B D=c \cos B \text { and } D C=b \cos C \\
& \text { and, } B C=a=B D+D C \\
& =c \cos B+b \cos C \\
& a=c \cos B+b \cos C
\end{aligned}
$
Similarly, other projection formulas can be derived
$
\begin{aligned}
& a=c \cos B+b \cos C \\
& b=c \cos A+a \cos C \\
& c=b \cos A+a \cos B
\end{aligned}
$
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Mathematics for Joint Entrance Examination JEE (Advanced) : Trigonometry
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