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Put cos²x = 1-sin²x
sin²x+(1-sin²x)²
= sin²x + 1 -2sin²x +sin⁴x
= 1 - sin²x + sin⁴x
= 1 - sin²x(1-sin²x)
= 1 - sin²x.cos²x
=1 - (sinx.cosx)²
= 1 - ¼(2sinx.cosx)²
2sinx.cosx = sin(2x)
= 1 -¼(sin²(2x))
Now to find range.
We know that range of sin(x) is from -1 to 1
-1 ≤ sin(x) ≤ 1
-1 ≤ sin(2x) ≤ 1
0 ≤ sin²(2x) ≤ 1
0 ≤ ¼sin²(2x) ≤ ¼
-¼ ≤ -¼sin²(2x) ≤ 0
1-¼ ≤ 1-¼sin²(2x) ≤ 1+0
¾ ≤ 1 - ¼sin²(2x) ≤ 1
¾ ≤ 1 A ≤ 1 (Proved)
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