LHS `=tan 70^(@)=tan(20^(@)+50^(@))=(tan20^(@)+tan 50^(@))/(1-tan20^(@)tan.50^(@))`
or `tan70^(@)-tan20^(@)tan50^(@)tan70^(@)=tan20^(@)+tan50^(@)`
or `tan70^(@)= tan70^(@)tan50^(@)tan20^(@)+tan20^(@)+tan50^(@)=2tan 50^(@)+tan20^(@)`
`=cot70^(@)+2cot40^(@)=RHS`.