Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 11

# If a = B Cos 2 π 3 = C Cos 4 π 3 Then Write the Value of Ab + Bc + Ca. - Mathematics

Short Note

If a = b $\cos \frac{2\pi}{3} = c \cos\frac{4\pi}{3}$ then write the value of ab + bc + ca.

#### Solution

$a = b \cos120°= c \cos 240°$
$\Rightarrow a = - \frac{1}{2}b = - \frac{1}{2}c$
$\text{ Therefore },$
$ab + bc + ca = \frac{- 1}{2}b \times b + b \times b + b \times \frac{- 1}{2}b$
$= - b^2 + b^2$
$= 0$

Is there an error in this question or solution?

#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 7 Values of Trigonometric function at sum or difference of angles
Q 9 | Page 27