L.H.S. = tan 48° tan 16° tan 42° tan 74°
= tan 48°. tan 16° . tan(90° – 48°) . tan(90° – 16°)
= tan 48° . tan 16° . cot 48° . cot 16° [∵ tan (90 – θ) = cot θ]
= tan 48° . tan 16° . \(\frac{1}{tan \,48
^∘}\) . \(\frac{1}{tan \,16
^∘}\) [∵ cot θ = \(\frac{1}{tan \,θ}\)]
= 1 = R.H.S.
∴ L.H.S. = R.H.S.