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Prove that: sin(-420^0)(cos390^0)+cos(-6...

Prove that: `sin(-420^0)(cos390^0)+cos(-660^0)(s in330^0)=-1.`

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`L.H.S.=sin(-420^@)(cos390^@)+cos(-660^@)(sin330^@)`
`=-sin420^@cos390^@+cos660^@sin330^@`
`[ :' sin(-theta)=-sintheta,cos(-theta)=costheta]`
`=-sin(90^@xx4+60^@)cos(90^@xx4+30^@)`
`+cos(90^@xx7+30^@)sin(90^@xx3+60^@)`
`=-(sin60^(@))(cos30^@)+(sin30^@)(-cos60^@)`
`=-sqrt3/2xxsqrt3/2+1/2(-1/2)=-1=R.H.S.`
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CENGAGE ENGLISH-TRIGONOMETRIC FUNCTIONS -SINGLE CORRECT ANSWER TYPE
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