# sina*cosa=4/7 , a=?

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• Oct 29th 2008, 12:59 PM
i.s.tzortzis
sina*cosa=4/7 , a=?
sina*cosa=4/7 , a=?
• Oct 29th 2008, 01:42 PM
skeeter
$\sin{a} \cos{a} = \frac{4}{7}$

$2\sin{a} \cos{a} = 2 \cdot \frac{4}{7}$

$\sin(2a) = \frac{8}{7}$

there is no solution for "a" to satisfy this equation ... why?
• Feb 2nd 2010, 09:54 PM
afsaneh
answer
sin2a<1 then 2sina.cosa<1 so sina.cosa<1/2
4/7 is not less than 1/2 and equation is not true.
• Feb 2nd 2010, 10:50 PM
pacman
sin 2A
see the graph . . . . no roots possible; sin A cos A = 4/7. The red and blue wont meet . . . .

http://www1.wolframalpha.com/Calcula...image/gif&s=43

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