We are asked to prove the trigonometric identity $(1 + tan A)^2 + (1 + cot A)^2 = (sec A + csc A)^2$.

AlgebraTrigonometric IdentitiesProofTrigonometry
2025/4/15

1. Problem Description

We are asked to prove the trigonometric identity (1+tanA)2+(1+cotA)2=(secA+cscA)2(1 + tan A)^2 + (1 + cot A)^2 = (sec A + csc A)^2.

2. Solution Steps

We will start by expanding the left-hand side (LHS) of the equation:
(1+tanA)2+(1+cotA)2=(1+2tanA+tan2A)+(1+2cotA+cot2A)(1 + tan A)^2 + (1 + cot A)^2 = (1 + 2 tan A + tan^2 A) + (1 + 2 cot A + cot^2 A)
=2+2tanA+2cotA+tan2A+cot2A= 2 + 2 tan A + 2 cot A + tan^2 A + cot^2 A
We know the following trigonometric identities:
tanA=sinAcosAtan A = \frac{sin A}{cos A}
cotA=cosAsinAcot A = \frac{cos A}{sin A}
secA=1cosAsec A = \frac{1}{cos A}
cscA=1sinAcsc A = \frac{1}{sin A}
sin2A+cos2A=1sin^2 A + cos^2 A = 1
tan2A+1=sec2Atan^2 A + 1 = sec^2 A
cot2A+1=csc2Acot^2 A + 1 = csc^2 A
From the Pythagorean identities, we can replace tan2Atan^2 A and cot2Acot^2 A in our expression:
tan2A=sec2A1tan^2 A = sec^2 A - 1
cot2A=csc2A1cot^2 A = csc^2 A - 1
Substituting these into our expression, we get:
2+2tanA+2cotA+tan2A+cot2A=2+2tanA+2cotA+(sec2A1)+(csc2A1)2 + 2 tan A + 2 cot A + tan^2 A + cot^2 A = 2 + 2 tan A + 2 cot A + (sec^2 A - 1) + (csc^2 A - 1)
=2+2tanA+2cotA+sec2A1+csc2A1= 2 + 2 tan A + 2 cot A + sec^2 A - 1 + csc^2 A - 1
=2tanA+2cotA+sec2A+csc2A= 2 tan A + 2 cot A + sec^2 A + csc^2 A
Now, let's expand the right-hand side (RHS) of the equation:
(secA+cscA)2=sec2A+2secAcscA+csc2A(sec A + csc A)^2 = sec^2 A + 2 sec A csc A + csc^2 A
Now, we need to show that 2tanA+2cotA=2secAcscA2 tan A + 2 cot A = 2 sec A csc A.
Let's rewrite 2tanA+2cotA2 tan A + 2 cot A:
2tanA+2cotA=2sinAcosA+2cosAsinA=2(sin2A+cos2AsinAcosA)=2(1sinAcosA)2 tan A + 2 cot A = 2 \frac{sin A}{cos A} + 2 \frac{cos A}{sin A} = 2 (\frac{sin^2 A + cos^2 A}{sin A cos A}) = 2 (\frac{1}{sin A cos A})
Since secA=1cosAsec A = \frac{1}{cos A} and cscA=1sinAcsc A = \frac{1}{sin A}, then secAcscA=1cosAsinAsec A csc A = \frac{1}{cos A sin A}.
Therefore, 2secAcscA=2sinAcosA2 sec A csc A = \frac{2}{sin A cos A}.
So, 2tanA+2cotA=2secAcscA2 tan A + 2 cot A = 2 sec A csc A.
Thus, the left-hand side is 2tanA+2cotA+sec2A+csc2A=2secAcscA+sec2A+csc2A2 tan A + 2 cot A + sec^2 A + csc^2 A = 2 sec A csc A + sec^2 A + csc^2 A.
And the right-hand side is sec2A+2secAcscA+csc2Asec^2 A + 2 sec A csc A + csc^2 A.
Therefore, (1+tanA)2+(1+cotA)2=(secA+cscA)2(1 + tan A)^2 + (1 + cot A)^2 = (sec A + csc A)^2 is true.

3. Final Answer

(1+tanA)2+(1+cotA)2=(secA+cscA)2(1 + tan A)^2 + (1 + cot A)^2 = (sec A + csc A)^2

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