If cosA=4/5. Find cos2A?

2 Answers
maganbhai P.
Apr 22, 2018

#cos2A=7/25#

Explanation:

Here,

#cosA=4/5 > 0=>I^(st)Quadrant or IV^(th) Quadrant#

We know that,

#cos2A=2cos^2A-1=2(4/5)^2-1#

#:.cos2A=2xx16/25-1=32/25-1#

#=>cos2A=(32-25)/25=7/25 > 0#

#=>cos2A=7/25#

Jim G.
Apr 22, 2018

#cos2A=7/25#

Explanation:

#"using the "color(blue)"trigonometric identity"#

#•color(white)(x)cos2A=cos^2A-sin^2A#

#rArrcos2A=2cos^2A-1#

#rArrcos2A=2xx(4/5)^2-1#

#color(white)(rArrcos2A)=32/25-25/25=7/25#

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