Rút gọn và tính giá trị biểu thức sau:

C = sin810° cos540° + tan135° cot585°

D = sin825° cot ( 15°) + cos75° sin ( 555°)

`C = sin810^\circ . cos540^\circ + tan135^\circ. cot585^\circ`

`= sin( 90^\circ + 2.360^\circ ). cos( 180^\circ + 360^\circ ) +``tan( -45^\circ + 180^\circ )` `. cot( 45^\circ + 3.180^\circ )` 

`= sin90^\circ . cos180^\circ - tan45^\circ . cot45^\circ`

`= 1. ( -1 ) - 1 . 1`

`= -2`

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`D = sin825^\circ . cot ( - 15^\circ) + cos75^\circ sin . ( - 555^\circ)`

    `= [\sqrt{6} + \sqrt{2}]/[4] . [\sqrt{6} + \sqrt{2}]/[4] + [\sqrt{6} - \sqrt{2}]/[4] . [\sqrt{6} - \sqrt{2}]/[4]`

    `= ([\sqrt{6} + \sqrt{2}]/[4])^2 + ([\sqrt{6} - \sqrt{2}]/[4])^2`

    `= 1`