The value of tan⁡6∘tan⁡42∘tan⁡66∘tan⁡78∘, is

We have,tan⁡6∘tan⁡42∘tan⁡66∘tan⁡78∘=tan⁡6∘tan⁡60∘−18∘tan⁡60∘+6∘tan⁡60∘+18∘=tan⁡6∘tan⁡60∘+6∘tan⁡18∘tan⁡60∘−18∘tan⁡60∘+18∘tan⁡18∘=tan⁡6∘tan⁡60∘+6∘tan⁡3×18∘tan⁡18∘=tan⁡6∘tan⁡60∘−6tan⁡60∘+6tan⁡18∘=tan⁡18∘tan⁡18∘=1