1. sin4+cos4=1-2(sin2×cos2)
2. sin6+cos6=1-3(sin2×cos2)
3. 1-2(sin×cos)=(sin-cos)2
4. tg ×cot =1
5. sec=1/cos → sec2=1/cos2=1+tg2
6. csc=1/sin → csc2=1/sin2 =1+cot2
7. sin2+cos2=1 → sin2=1-cos2 → cos2=1-sin2
8. tan+cot=1/(sin×cos)=sec ×csc
9. sin(a ± b)=sin(a)cos(b)±sin(b)cos(a)
10. cos(a+b)=cos(a)cos(b) - sin(a)sin(b)
11. cos(a - b)=cos(a)cos(b)+sin(a)sin(b)
12. cos(a - b)×cos(a +b)=cos2a - sin2b
13. sin(a +b)×sin(a - b)=sin2a - sin2b
14. tan(a+b)=( tan(a) + tan(b) ) / ( 1-tan(a)×tan(b) )
15. tan(a - b)=( tan(a) -tan(b) ) / ( 1+tan(a)×tan(b) )
16. cot(a+b)=( cot(a)×cot(b) - 1)/( cot(a)+cot(b) )
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