20. Which of the following is not an identity? a) sin2 a+cos2 a = 1
b) sin a = tan a * cos a
c) 1 + cot2 a = csc2 a
d) 1 - sec2 a = tan2 a

Choice D)  1 - sec^2(a) = tan^2(a) is not an identity.

Explanation:

a) is the pythagorean trig identity assuming you meant to write sin^2(a)+cos^2(a) = 1

b) is a rewritten version of tan = sin/cos. You multiply both sides by cos(a).

c) is an identity that you can find through dividing everything in equation (a) by sin^2. I'm assuming you meant to put exponent symbols before each "2".

d) is not an identity. I recommend looking at a table or graph that compares the two sides as separate functions. You should see they are not the same. The actual identity should be sec^2(a) - 1 = tan^2(a). You divide both sides of equation (a) by cos^2 and do a bit of algebra to get it into this form.

step-by-step explanation:

this is not a question

a.

step-by-step explanation:

step 1 distribute 2 to every term in the parentheses

step 2 add 10 on both sides of the equal sign