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About the problem of compulsory four in senior high school mathematics! ! Come and help me. Thank you! ! ! Urgent! ! enclose herewith
-(sin 20 cos 70+cos 20 sin 70)=-sin(20+70)=- 1

For details of this formula, please refer to P39, the first volume of senior high school mathematics (the second volume).

The sign problem of trigonometric function sinxcsxcx is positive in quadrant one and negative in quadrant three and quadrant four.

Cosx secx-4 quadrant is positive and 23 quadrant is negative.

Tanxcottx-3 quadrant is positive and 24 quadrant is negative.

Please ask more questions about trigonometry. I thought trigonometry was awesome at that time.

Hope to adopt!

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