Answer:
Step-by-step explanation:
18.
LHS
[tex]\frac{cos^{2}a-sin^{2}b }{sin^{2}a.sin^{2}b}[/tex]
= [tex]\frac{cos^{2}a }{sin^{2}a.sin^{2}b} -\frac{sin^{2}b }{sin^{2}a.sin^{2}b}[/tex]
= [tex]\frac{cos^{2}a}{sin^{2}a}[/tex] × [tex]\frac{1}{sin^{2}b}[/tex] - [tex]\frac{sin^{2}b}{sin^{2}b}[/tex] × [tex]\frac{1}{sin^{2}a}[/tex]
= [tex]cot^{2}a[/tex] × [tex](1+cot^{2}b)[/tex] - 1 × [tex](1 + cot^{2}a)[/tex]
= [tex]cot^{2}a[/tex] + [tex]cot^{2}a.cot^{2}b[/tex] - 1 - [tex]cot^{2}a[/tex]
= [tex]cot^{2}a.cot^{2}b[/tex] - 1
Then you can conclude that LHS = RHS
19. It's weird. I tried a few ways like simplifying, dividing both numerator and denominator on LHS by tan or sin or cos but it didn't work out.
