Question

2cos10° × cos20° can be written as..​
[cos(__)cos(20-10)]
tell what is in those dashes

Answer 1

Step-by-step explanation:

cosAcosB = (1/2)[cos(A+B) + cos(A-B)]

2 x cos10° × cos20° = cos(10+20) + cos(10-20)

=> cos(30) + cos(20-10)

Note that cos(-X) = cos(X)

Answer 2

$\huge \underline \bold \color{plum} \mathcal{HEYA }$

$2 \cos(10) \cos(20)$

Using the formula

$2 \cos(a) \cos(b) = \cos(a + b) + \cos(a - b)$

We get

$\cos (10 + 20) + \cos(10 - 20)$

$\therefore \: the \: answer \: is \color{red}(10 + 20$