Question

cosa=3/5,cosb=4/5to cos1/2(a-b) ka man btao

Answer 1

cosa=3/5,

then sina=
$\sqrt{1 - cos {a}^{2} }$
now
(1-9/25)^1/2

(16/25)^1/2

4/5= sinA

now
cosb=4/5
then
sinb = 3/5

then cosa-b= cosacosb+sinasinb
= 3/5×4/5+4/5×3/5
= 12/25+12/25
=24/25



then cos1/2(a-b)=1/2×24/25

=12/25...................ans

Answer 2

cosa=3/5
a=66.4°
cosa=4/5
b=36.9°
then cos1/2(a-b)= cos1/2(66.4-36.9)
=cos1/2(29.5)
=cos(14.75)
=0.967