Select a suitable identity and find the following products(i) (3t + 9s

1.	Select a suitable identity and find the following products(i) (3t + 9s) (3t – 9s) (ii) (kl – mn) (kl + mn)
Answer» i) (3t + 9s) (3t – 9s) = (3t)² – (9s)² [ ∵ (a + b) (a – b) = a² – b² ] = 3t × 3t – 9s × 9s = 9t² – 81s² ii) (kl – mn) (kl + mn) = (kl)² – (mn)² [ ∵(a + b) (a – b) = a² – b² ] = kl × kl – mn × mn = k²l² – m²n²

Select a suitable identity and find the following products(i) (3t + 9s) (3t – 9s) (ii) (kl – mn) (kl + mn)

Answer»

i) (3t + 9s) (3t – 9s) = (3t)² – (9s)² [ ∵ (a + b) (a – b) = a² – b² ]

= 3t × 3t – 9s × 9s

= 9t² – 81s²

ii) (kl – mn) (kl + mn) = (kl)² – (mn)² [ ∵(a + b) (a – b) = a² – b² ]

= kl × kl – mn × mn

= k²l² – m²n²