Question

(1除以cos80平方-1除以cos10平方)乘以 1除以cos20

Answer 1

原式=[1/cos^2 (90-10) -1/(cos10)^2]× 1/cos20

=[1/(sin10)^2 -1/(cos10)^2]×1/cos20

=[(cos10)^2-(sin10)^2 / (sin10cos10)^2]× 1/cos20

=[cos20 /(1/2 sin20)^2]× 1/cos20

=1/(1/4)(sin20)^2

=4/(sin20)^2

≈34