y = sec^2theta + cos^2theta, theta != 0, then?(Solve it without A.M - G.M inequality method)

A: y = 0
B: y <= 2
C: y >= -2
D: y > 2
The answer is (D) for your reference

1 Answer
Aug 2, 2017

y>=2

Explanation:

y=sec^2theta+cos^2theta

=sec^2theta-2secthetacostheta+cos^2theta+2

=(sectheta-costheta)^2+2

(as 2secthetacostheta=2)

as (sectheta-costheta)^2>=0

(sectheta-costheta)^2+2>=2

i.e. y=sec^2theta+cos^2theta>=2