Question

A is acute and cosA=4÷5 then cos2A​

Answer 1

Step-by-step explanation:

we kow that cos37 = 4/5

so cos 2A = cos (37+37)

= cos^2 (37) - sin^2(37)

= 16÷25 - 9÷25

= 7÷25

Answer 2

Step-by-step explanation:

Given,

cos(a)= 4/5

As we know,

cos 2a = cos^2(a) - sin^2 (a)

Also,

sin^2(a) + cos^2(a) =1

sin^2(a) =1 - cos^2(a)

sin^2(a) = 1 - (4/5)^2

sin^2(a) = 1 - 16/25

sin^2(a) = 25-16/25

sin^2(a) = 9/25

sin^2(a) = 9/25.................(1)

cos^2(a) = 16/25..............(2)

Therefore,

cos2a = 16/25 - 9/25.........(from 1 & 2)

cos2a = 7/25

Answer : cos2a = 7/25